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Chris Harris Jr. was not supposed to be here.

He was not supposed to be charging out of a tunnel in an NFL stadium to the roar of thousands, let alone starting on the league's third-ranked defense. Less than one year removed from ACL surgery, he was not supposed to be able to burst and cut and change directions the way elite athletes can.

He was not supposed to have a passer rating against him of 47.8—second best in the NFL, according to Pro Football Focus (subscription required). He was not supposed to be trying to deny Colts wide receiver T.Y. Hilton of the football and his dreams in a critical matchup Sunday.

Yet here he is, all 5'9" of him, an imposing figure in the AFC playoffs and a reason for Broncos fans to believe.

The unlikely story of Chris Harris became the impossible story of Chris Harris last January 12. It was the third quarter of a divisional playoff game against the Chargers. The wide receiver being covered by Harris ran a little comeback route. Harris broke on the play like he has done a thousand times before. Without ever being touched, Harris told Bleacher Report, he felt a sharp pain in his left knee and went down. He got up but couldn't put pressure on the leg, so he skipped and hopped and limped his way off the field.

A subsequent appointment with knee specialist Dr. James Andrews showed Harris had partially torn his ACL, and he was no more than a good sneeze away from completely tearing the ligament.

Before that appointment, Harris was a medical insurance company's dream. He never had been injured. Not in his previous 50 NFL games. Not in his 50 games at Kansas. And not in high school or grammar school. He said he had not missed a game or even a practice in his life because of an injury.

He would miss his first game January 19, the AFC Championship Game against the Patriots. Then came Super Bowl week. The Broncos left Denver for New Jersey the Sunday before the game, but Harris and the other injured players had to stay behind. They traveled with wives and family members on Wednesday. After leading the defense in snaps in the regular season, Harris was left out of the Super Bowl team photo.

"There was nothing I could do or say to make him happier," his wife Leah Harris said.

As Harris watched the Seahawks crush the Broncos from the sideline at MetLife Stadium, he was overcome by helplessness and frustration. Surely, he could have made a difference. "The worst feeling ever," he said. "My low point."

Gregory Bull/Associated Press

Four days later in Pensacola, Florida, Harris was lying in an uncomfortable bed wearing a johnny and a fluffy, light-blue cap, and surrounded by strangers whose faces were obscured by surgical masks. Some people freak out there. Once that skin is cut, they sometimes say, that knee never will be the same. But that wasn't what Harris was thinking about as he was waiting for the anesthesia to kick in. He was thinking about next September. "It was on my mind to be starting on opening day," he said.

Sure it was. His chances, however, seemed to be only slightly better than his chances of hitting the Powerball lottery. The rule of thumb always has been it takes about a year to come back from this kind of surgery. The fastest previous NFL return from an ACL reconstruction is believed to be by Vikings running back Adrian Peterson in eight months. And that comeback still was considered almost mythical. Harris was trying to come back in seven months.

His rehab started the next day, with simple exercises to bend the knee. Leah Harris still has an image in her mind of the pain in her husband's eyes as he tried to stretch the joint. 'Niners linebacker NaVorro Bowman, who had ACL surgery around the same time, was his post-op companion, as the two competed to see who could stretch their repaired knees the farthest.

Seven days after surgery at the rehab center in Florida, Leah challenged her husband to lay down his crutches and take his first step. Leah recorded the steps on video, and the proud wife posted the milestone on her Instagram page.

"I kept telling him, 'You are going to be OK,' and I believed it in my heart of hearts," she said. When he showed progress, she would say, "That's my Pitbull," using the nickname his teammates gave him.

Throughout the process, Leah served as caretaker/cheerleader/coach for her husband. From sponge baths to stopwatches, she was all-purpose. For every rehab session, every workout, his dutiful better half was there with video recorder in hand. Afterward, Harris would watch the tapes with Leah and his agent Fred Lyles to gauge his progress. She taped so many hours, she filled up her computer's memory and had to buy a new one.

Harris said Dr. Andrews told him missing one day of rehab could set him back two weeks. So he was determined not to miss one second. Lyles said he and Harris spoke four times daily during his rehab, and Harris was working on his knee "almost around the clock." There was strength training, ice, heat, electronic stimulation, massage and an active compression and cold therapy machine that he strapped on his leg called Game Ready.

Broncos linebacker Von Miller had his ACL repaired about a month before Harris, and the two worked with one another in the offseason under the direction of Broncos trainer Steve "Greek" Antonopulos.

Miller told Broncos.com:

We did just about everything together. We were doing the exact same thing for the last six weeks of the treatment. We were together and it was great. Chris, he's a fierce competitor and working out with him, he pushed me. I like to think I pushed him too.

Joe Mahoney/Associated Press

Performance-enhancers, Harris said, never were a consideration. He did think about stem cell therapy, in part because Miller was undergoing the controversial treatment. He decided against it because he didn't want to miss a moment of rehab.

"Having stem cell would mean doing nothing for two weeks," he said. "I was on a tight schedule and had to stick to it. If I had missed two weeks of rehab, I probably would not have been ready for the first game."

Two months after surgery, he ran for the first time.

"It was a totally different feeling, like teaching your leg how to run all over again," he said. "I had to learn the form, teach the leg to move again."

By April, Harris' rehab reached another level, with more running, aggressive lifting, agility training and football drills. For six hours daily, he would sweat and strain. Harris caught 150 balls daily and never missed a day. He pushed the pace however he could.

It wasn't surprising that Harris was not about to conform to conservative rehab guidelines. He never has let the expectations of others dictate his destiny.

After an All-State senior year at Bixby High School in Bixby, Oklahoma, Harris received only one scholarship offer. Despite arriving in Lawrence, Kansas, with little fanfare, Harris started 41 games for the Kansas Jayhawks and made more tackles than all but two other players in school history.

His senior year, Harris volunteered to switch to safety from cornerback because the team had problems at safety. NFL teams subsequently evaluated Harris as a safety, but scouts said he was too small to get a serious look. Harris was not invited to the scouting combine.

Undaunted, Harris put together a personal highlight tape. He sent a copy to Lyles, who was impressed and agreed to represent him. In order to prepare for his pro day workout, Harris drove 80 miles three times a week to work with a trainer at Emporia State, Lyles said. At his pro day, Harris ran a 4.48 in the 40-yard dash.

That wasn't enough to get him drafted, though. And for a time, it appeared he would not merit a free-agent invitation. Only the Broncos were remotely interested. The story, told to ESPN.com's Jeff Legwold, goes that they had one roster spot for a cornerback, and there were three players on their board. Denver scouts told head coach John Fox he could have the fast one, the big one or the smart one who started four years at Kansas. Fox chose the smart one, and the Broncos offered Harris a $2,000, take-it-or-leave-it bonus.

Harris began training camp as the longest of long shots. It was two days into practice when the Broncos figured out Harris was better than they thought he was. Harris would volunteer to go to the front of the line for one-on-one reps so he could have a chance to cover Broncos star wide receivers Brandon Lloyd and Demaryius Thomas. And he would hold his own. The great Champ Bailey had seen enough then to tell Legwold that Harris would be playing in the Broncos secondary long after he was retired.

Four years later, Harris again was a long shot to be on the field on opening day. He went through the offseason not participating in any practices, and he was mostly a spectator in training camp. Harris sat out during preseason too. Then, during an August visit to Dr. Andrews, Harris passed every test and received clearance to start playing again.

Ed Andrieski/Associated Press

He wore a knee brace in practice but said he found it cumbersome. When the Broncos opened the season against the Colts, Harris would be in the starting lineup, and he would not be wearing a brace.

He acknowledges being a little fearful that day. Then, in the first quarter, Hilton ran a slant. Without thinking, Harris planted on his left leg to jump the route and broke up the pass from Andrew Luck. "That's when I realized my leg was good," he said.

He meant relatively good. His knee was very sore after the game, and his leg was fatigued. The left leg remained smaller than the right until October. But Harris was effective on the field from the first snap, and he progressed steadily. About a month ago, Harris and Leah rewatched some of the offseason rehab videos.

"Look at how far you've come," she told him. Hilton is likely to see how far Harris has come at Mile High Stadium on Sunday.

At a December press conference to announce Harris had agreed to a five-year, $42.5 million contract extension, Broncos general manager John Elway said Harris has, "probably been the fastest guy to ever come back off an ACL." No one argued.

"You look at what he did, the time that he spent this offseason, to get healthy and be ready to go, and be ready to start the regular season," Elway said. "I'm not sure anybody has ever come back as fast as he's come back, so that kind of tells you what kind of guy he is."

It was long ago when Harris figured out hard work could lead him to a better place. When he was five, his parents divorced. He grew up in a single-parent home, the oldest child feeling like he should be the man of the house. His mother, Lisa Goff, delivered mail during the day and worked a second job on the side. Harris wanted a better life. He knew in order to make his dream reality, he would have to push himself beyond where others were willing to go.

That's how he ended up at Kansas. That's how he made the Broncos as an undrafted rookie. And that's how in December he became the first undrafted cornerback to make a Pro Bowl with his original team in 33 years.

Harris didn't just come back quicker than anyone. He came back better. According to STATS, Harris was thrown at 89 times. Opponents caught just 39 of those attempts—43.8 percent. He had 18 passes defensed, tied for fourth most in the league, and he didn't allow a single touchdown. He also played 967 snaps.

He might be the Comeback Player of the Year. Or maybe even the comeback player of all time.

Elaine Thompson/Associated Press

"He believed he could come back bigger, faster, stronger, and then he made it happen," Lyles said.

Broncos defensive coordinator Jack Del Rio said this has been the best year of Harris' career.

"Each year, he has gotten a little better," Del Rio said. "Even coming off major reconstruction, he has improved. It's amazing."

Also amazing has been Harris' versatility. It's not like there has been an aspect of his game that has been diminished by his knee.

In a December game, Bills quarterback Kyle Orton found himself face-down on the turf with Harris' arms wrapped around his legs. Harris blitzed off the right corner, went inside on left tackle Cordy Glenn and sacked Orton from behind.

Harris lines up on the right and left side, and sometimes is matched up with a receiver, as he recently was with Keenan Allen of the Chargers. He regularly plays over the slot on nickel. That's where he was playing when he picked off a Russell Wilson pass intended for Percy Harvin last September. He even occasionally lines up as a safety.

"He can play anywhere," Del Rio said. "All the things you ask him to do at all the spots, he can do it. I think he's probably underrated as an athlete. And his tenacity and preparation and intelligence, you add it up and it's a heck of a package."

Harris believes he is a better cornerback because of his knee problems. He thought his knee would benefit if he played at a lighter weight, so he dropped 12 pounds to 188, and it has made a difference.

"I feel faster this year," he said. "I feel more explosive. That is what has elevated my game to another level. I guess it teaches you to run properly. It fixes all the bad mechanics in running and the things you were doing wrong technique-wise."

Bailey told Legwold he believes Harris is playing better than any corner in the league. He also said Harris reminds him of himself in the way his technique is similar to Bailey's.

Harris does not have Bailey's special physical traits, but he studied Bailey and imitated his approach. And after Bailey left Denver in the offseason, Bailey's locker was given to Harris. It was a symbolic gesture, but also a practical one, as now Harris is in the center of the room where he can be seen and heard by all.

"It was a great situation for Chris to come in as a young player and have Champ be the example in terms of work ethic and how you prepare, how you get yourself ready each week, refine your technique each day," Del Rio said. "Now Chris is providing that for our younger corners."

Once unwanted, Chris Harris now sits on the throne that belonged to the best cornerback of a generation. Who could have known he was supposed to be here?

Dan Pompei covers the NFL for Bleacher Report. Follow him on twitter @danpompei.