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LAKE FOREST, Ill. — It was one of the most important plays of the 2015 season. Coming off two straight wins on the road, the Bears were within reach of a 5-5 record — needing just a two-point conversion on Nov. 22 to tie the future Super Bowl champion Denver Broncos with 24 seconds left in regulation.

Jay Cutler had just led his offense 65 yards down the field with ease — against the league’s top defense — and the under-appreciated quarterback was on the fringe of taking a lost season and thrusting the Bears right into the middle of a playoff race.

All they needed was a two-point conversion to tie. But before the play ever really got started, it blew up.

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Cutler checked from a pass to a run on the same play the Bears ran successfully for two points two weeks prior in San Diego. The Broncos were showing a different look than the Chargers, but Cutler believed the Bears still had the blocking numbers to hand the ball off to rookie running back Jeremy Langford and head to overtime.

The only problem? One player didn’t get the check. And it was an important player. The center. Rookie Hroniss Grasu.

Grasu snapped the ball to Cutler and jumped backward out of his stance to pass protect. With the rest of the offensive line moving forward, left guard Matt Slauson and left tackle Charles Leno Jr. were forced to compensate and safety T.J. Ward came unblocked off the left edge to blow up the play in the backfield. Langford never had a chance.

In hindsight, the numbers never seemed to be in the Bears favor to run the ball, but the play also never had a chance with Grasu missing the check. Of all people to miss a check, it should never be the center.

“Last year I was thinking just way too much,” Grasu told WGN Radio Wednesday. “It’s a bad thing to do, but I’m glad it happened because it definitely humbled me.”

While many blamed Grasu’s lack of strength for his rookie struggles, the center was more worried about the mental side of the game — the part that was never really an issue for him in college.

“I pride myself on being smart and having a good knowledge of the playbook and I was nowhere near where I needed to be last year,” he admitted.

As has been well documented, the third-round pick was thrown onto the field earlier than planned as a rookie. Grasu was drafted to take over the center position from Roberto Garza, but the Bears signed Will Montgomery as a one-year stop-gap while Grasu developed physically. Things changed quickly, however, when Montgomery broke his leg in Week 4 against the Raiders. Suddenly, Grasu had to play.

Unfortunately, after just two starts, Grasu suffered a neck injury and missed the next three games. As it turned out, the Week 11 game against the Broncos was his first live football action in over a month and just his third NFL game. Also relevant: Grasu wasn’t on the field when the Bears successfully ran that same two-point conversion play two weeks earlier. He was out with the neck injury.

The NFL isn’t fair and excuses are (mostly) worthless, but there are at least some reasonable explanations for why Grasu struggled as a rookie in eight starts. The way the Bears look at it: there’s reason for optimism in Year 2.

That’s good, because Grasu is the most important domino on an offensive line that is far from set. Bears general manager Ryan Pace brought in reinforcements, signing veterans Manny Ramirez and Ted Larsen in case Grasu doesn’t make the necessary adjustments in Year 2, but make no mistake: the center position is Grasu’s to lose and the Bears will be better — and deeper — at both center and guard if Grasu wins the job.

For what it’s worth, Grasu did add 10 pounds of muscle in the offseason, but the Bears won’t know the payoff of the added strength until the pads go on in training camp.

“My weight has gone up, but it’s all good weight, it’s all strength,” Grasu said. “I don’t feel any different than last year. I added about 10 pounds or so, but I don’t feel any different. I feel just as fast, I feel just as quick.”

The truth is, adding weight was the easy part. Grasu’s biggest offseason focus was in the playbook.

“I found one thing every single week to get better in the playbook, found one concept in the playbook and worked on that all week,” Grasu said. “I just got it down to where I don’t have to think about it anymore on the field. Now it’s really been paying off because I’m playing faster, I’m more confident.”

It will be interesting to see how Grasu stacks up physically this season, but the mental improvements should be impactful. They may even be the difference between a win and loss.

The Bears were a 6-10 football team in 2015, but there’s many inside Halas Hall who believe they weren’t that far away from being a 10-6 team. The two-point conversion against the eventual Super Bowl champion Broncos serves as evidence to that line of thinking.

Grasu doesn’t need to transform into a Pro Bowler for the Bears to be improved this season, but he does need to be more reliable, both physically and mentally. If he is, he shouldn’t have a problem winning the starting center job and the entire offensive line will be better off.

Adam Hoge covers the Chicago Bears for WGN Radio and WGNRadio.com. He also co-hosts The Beat, weekends on 720 WGN. Follow him on Twitter at @AdamHoge.