Roquan Smith's second season is over with three weeks remaining after he tore a pectoral muscle Thursday night in the victory over the Dallas Cowboys, Matt Nagy confirmed Monday.

Smith, the Bears' leading tackler, is the second starting inside linebacker and fourth defensive starter overall to suffer a serious injury this season. Running mate Danny Trevathan hasn't played since dislocating his elbow on the first defensive series in Week 10, and fellow Pro Bowler Akiem Hicks hasn't played because of a similar elbow injury since Week 5, though he's expected to return Sunday against the rival Packers in his first game eligible to be activated from short-term IR.

Smith, the eighth overall pick in last year's draft who came within three tackles of breaking Hall of Famer Brian Urlacher's rookie franchise record, had played his best football as a sophomore since Trevathan went down.

"Overall, I really thought you felt him — especially here in the last several games — really turning it on, really playing fast," Nagy said Monday, when the Bears held a light practice at Halas Hall to begin Week 15 preparations. "Every play that he gets, every game that he plays, for the rest of his career, he’s only [going to] get better and better in my opinion."

But it was an up-and-down encore after being named as a Pro Bowl alternate at the end of his rookie season. Smith missed the first game of his career in Week 4 because of a personal matter that prevented him from fully regaining his on-field footing until November. The Bears have remained mum on what specifically was affecting Smith, but his reduced playtime, if not the absence of his usual trademark speed and aggressiveness in the first few games upon returning, was evident.

Smith was certainly close to his top form in his last full game, the Thanksgiving victory in Detroit, where he notched a career-high 16 tackles and two sacks — including the game-sealer — while playing every defensive snap for the second consecutive week.

"I feel that way," Nagy said Monday when asked whether Smith had made it all the way back, after the coach admitted earlier in the season his prized backer wasn't playing as advertised. "I mean you see it on tape. He's really moving fast, flying around making tackles. He's always had instincts — that's been his biggest strength — and he's violent, he's a violent tackler, and I think now as we go here it's just going to be focusing in on having the game slow down, just like it would for a quarterback, slow down a little bit there in the middle of the field."

Smith was injured sometime during the Bears' opening defensive series, a 17-play, 75-yard TD drive by the Cowboys, who had success targeting the reigning Butkus Award winner in coverage wiith TE Blake Jarwin and in the run game with Zeke Elliott.

But the Bears 'D' was brilliant thereafter in the absence of both starting backers Thursday night, thanks in large part to the yeoman work of reserve Kevin Pierre-Louis, who had two passes defensed and a tackle for loss in a superlative sideline-to-sideline effort — easily his most extensive to date on 'D' with the Bears. Pierre-Louis took a page out of the Trevathan replacement handbook of Nick Kwiatkoski, another backup pressed into action and playing lights out in a contract year.

"We worked together a little bit in the preseason, which was great, and then OTAs, preseason practices, and our linebacking group as a corps are great people to be around," Pierre-Louis said Monday. "So, anybody that we're out there with, we're going to mesh together well."

With Trevathan, Kwiatkoski and Pierre-Louis all impending free agents, the Bears face difficult offseason decisions on finding the ideal complement to mesh with the elite skill set of the 22-year-old Smith, who may or may not be fully healthy when the new league year begins.

Trevathan is perhaps the defense's most established leader and was building on his best season with the Bears in 2018 prior to getting injured. He'll also be 30 in March, when he potentially could command at least one more lucrative multi-year contract. At least in theory, the presence of Smith should create leverage for the Bears, who surely drafted him thinking Trevathan was closer to Smith's floor than ceiling.

Meantime, the Bears have younger, more pressing in-house extension candidates, perhaps even at the same position.

Prior to this season, Kwiatkoski was a special-teams stalwart whose lack of speed was thought to be limiting on defense. Yet his contributions over the past month and a half have exceeded expectations and demonstrate clear improvements in coverage. Pierre-Louis, despite the barest track record, has all sorts of speed and versatility, which he began flashing Thursday and will now get to showcase in an enhanced role to finish the season.