"Instead of going into the [medical] tent there at the end [of the half], we did it inside with our trainer," Nagy said. "He got looked at, was in some pain, but we went through the whole thing. We knew he was good to go."

So good, in fact, that Trubisky engineered a 12-play, 80-yard touchdown drive on the Bears' first possession of the second half. He completed 7 of 9 passes for 66 yards, including a 14-yard back-shoulder TD to Tarik Cohen.

But Trubisky's injury worsened, and signs that it was affecting his play soon became evident. Nagy noticed that the quarterback wasn't using his body to step into his throws and not following through on his passes. Even on a draw play to Cohen, where the quarterback is supposed to first stand up before bending down to hand off the ball, Trubisky was unable to do so.

"What ended up happening over time with him is it became stiff, tighter and tighter, and that's where we ended up keeping an eye on it and noticing that," Nagy said. "You'll see it on almost every play. You'll notice where it's affected him. Whether it's a throw, whether it's a handoff, you'll see it, you'll notice it."

Backup Chase Daniel replaced Trubisky with 3:24 remaining in the game after the Rams had increased their lead to 17-7 on Malcolm Brown's 5-yard touchdown run. But Nagy insisted that the decision to make the change had been made before the Rams took a two-score lead.

The NBC television broadcast of the game captured what appeared to be an emotional conversation between Nagy and Trubisky on the Bears sideline.

"The thing that you love about Mitch is that he's extremely tough," Nagy said. "I love that about him. But at the same time, he's at a point where you could see that it was painful. So what I had to do was I wanted him to know coming from me that he needs to be brutally honest with me in regards to his pain and where he's at. And that's exactly what it was.

"I sat there and I told him, I said, 'Listen, man. We appreciate your toughness. We appreciate you being as tough as you can possibly be and want to stay in the game. But I need to be able to make a decision because there's a fine line of that pain or being injured to where it affects how you play or decisions that you make because of being injured.' And I think that's where we got to.

"Again, he's a tough, blue collar kid. And he's a fighter. But I wanted him to know that I fully support if he's in pain and it hurts to play and too much to where it's affecting our team, I need to make a decision. And that's exactly what that was."

Nagy wasn't surprised that Trubisky lobbied to stay in the game.

"He really wanted to," said the Bears coach. "I know him inside-out. I know how he is and I appreciate that about him. But sometimes I have to protect him from himself. That's exactly what that was."

Because the news of Trubisky's injury was not relayed to the broadcast booth, announcers Al Michaels and Cris Collinsworth speculated that the quarterback switch might have been made due to performance issues. But Nagy strongly refuted that idea.

"It had zero to do with his play," Nagy said. "It was completely based off of the injury that he had last night and where he's at, 100 percent."

Asked about Trubisky's status moving forward, Nagy said that the Bears are "working through that." He also revealed that Trubisky will "absolutely" remain the team's starting quarterback if he's healthy enough to play.

Nagy doesn't see a benefit in shutting down Trubisky if the third-year quarterback is able to continue to play.