THOUSAND OAKS, Calif. -- Sam Shields walked out of the postgame locker room with mixed emotions.

The Los Angeles Rams had just defeated the Minnesota Vikings to improve to 4-0, and Shields made his first start in more than two years. However, the one-time Super Bowl champion and Pro Bowl cornerback felt he could have performed better.

"Just being off for that amount of time and then coming in for a first full game, I did what I can," said Shields, who played all 74 defensive snaps for the Rams. "There was a couple mistakes."

Shields will get an opportunity for correction Sunday, when the Rams play the Seattle Seahawks (2-2) at CenturyLink Field.

It wasn't long ago that Shields doubted he would ever play football again.

An undrafted free agent who developed into a linchpin on the Green Bay Packers' defense for seven seasons, Shields took a knee to the head in the 2016 season opener and was sidelined for the remainder of the year, having suffered the fourth known concussion of his career.

The Packers released him in February 2017 and Shields spent the season away from football, as he suffered from headaches and nausea and even had difficulty going outside into the light.

"Throughout that year that I was out," Shields said, "I doubted a lot."

But determined to return to health, and perhaps even the field, Shields spent six months working with world-renowned neurologists at UCLA, and each day, Shields said, he felt a little bit better, until he reached the point where his head no longer caused him pain.

"I just took one day at a time and I felt better just out of nowhere," Shields said. "It was crazy, and I just started, you know, just like, 'Let's go workout.' I worked out, felt fine, felt fine that morning. I kept going after it and that's when I knew I finally was all right."

Doctors cleared Shields to play, and the Rams signed him to a one-year contract worth slightly more than $1 million.

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Through the first two games with the Rams, Shields, 30, played mostly on special teams. But after Aqib Talib suffered a high ankle sprain against the Los Angeles Chargers, Shields was thrust into action on defense and took over the starting role when Talib was put on injured reserve following surgery.

Against the Vikings, the 5-foot-11, 178-pound Shields finished with four tackles and a pass deflection. But he also gave up a 17-yard touchdown pass, and at times he struggled to cover Stefon Diggs and Adam Thielen, who finished with 123 and 135 receiving yards, respectively.

"The thing that says as much about Sam as anything is the way that he continued to compete," coach Sean McVay said. "If you're a corner, you're naturally going to be in spots where you get some situations where if a guy makes a play, how do you respond?"

Said defensive coordinator Wade Phillips: "His effort was tremendous. ... I think he could've gotten down a little bit mentally during the game, but he didn't. He kept fighting."

In a position group with Talib, Marcus Peters, Nickell Robey-Coleman and Troy Hill, Shields -- although quiet in demeanor -- has asserted himself as a veteran presence whom teammates can look to.

"I always asked him questions when he first got here, still ask him questions now," Robey-Coleman said. "So at the same time, we're playing together, but I'm learning from him."

Said Peters: "Ball skills, Shields, man. Cat-like movements."

Through four games, Shields has seven tackles, three pass deflections and an interception he returned 22 yards in the fourth quarter of a shutout victory over the Arizona Cardinals.

"You look at just his story, what he's come back from, his movement, different things like that, I think he's a resilient competitor," McVay said.

Playing opposite Peters -- who leads the NFL with 20 interceptions dating to 2015, Shields will be tested Sunday by quarterback Russell Wilson and a Seahawks receiving corps that features Tyler Lockett and potentially Doug Baldwin, who is nursing a knee injury.

Shields expressed confidence that he'll return to form.

"Being there full time, it was different. My body was different, my body got to get back to moving that way, you know, things like that, but it don't take that much time," Shields said. "I just have to keep coming in day after day and just keep grinding."

Shields has openly discussed the emotional challenges he has faced throughout each step of his comeback. None, he said, have caused him to waver on his decision to return to the game.

"I have a tough mind, I'm not going to lie," Shields said. "I guess that's just part, it's just a gift that I have and the most important part on your body is your mind, and I've been through a lot. I took time off and now I'm here."