Nagy stressed that Trubisky "won the job" over Nick Foles and it was not handed to him, adding that it was his call to make.
"This was going be a fair battle for both of these guys," Nagy said on a Zoom call with Chicago media. "Both these guys were really battling their tail off every single day.
"In the end, Mitch won the job, and I think that's very important for him, for us, for everybody to understand that he worked really hard to get to this point."
Nagy reiterated that Trubisky's footwork in the pocket improved over the summer, as did his accuracy and command.
"I thought Mitch really finished [training camp] strong, which was good," Nagy said. "To finish strong with some really accurate throws in the red zone. He did some things. I told you all along his footwork has been much better in the pocket, in regards to staying up in that pocket and hitching up there. Decision-making has been improved. So we're always working to try to get that better, and he knows that.
"You could feel the command. And I think the difference in the command that I felt, and that our coaches felt, was, it's one where it's very natural. It wasn't made up. It wasn't being told from somebody how to act. It was just very organic. And you can tell that with guys, when they are natural with that. I thought he improved there. For him, when he gets this opportunity, like he has right now, you could just feel it -- that he's ready to get out there and just really play hard for his teammates."
Trubisky's past success versus the Detroit Lions, the Bears' Week 1 opponent, played no role in the decision, Nagy said. Last season, Trubisky beat the Lions twice, throwing for 511 yards with six touchdowns and one interception in the two games.
Trubisky and Foles have been locked in a battle for the starting job throughout training camp, evenly rotating days on the first team since padded practices began Aug. 17.
In 2018, Nagy's first season as head coach, the Bears went 12-4 as Trubisky passed for 3,223 yards and 24 touchdowns with 12 interceptions and rushed for 421 yards and three scores.
Trubisky, while playing much of last season with a shoulder injury that required surgery in January, ranked 28th in the NFL in Total QBR (39.4), tied for 27th in touchdown passes (17), 21st in passing yards (3,138), 32nd in yards gained per pass attempt (6.1) and 28th in traditional quarterback rating (83.0) as the Bears finished 8-8.
"I control my own destiny, and I'm definitely not going to take this opportunity for granted," Trubisky said Sunday. "I feel like I got a fresh, new, clean slate. And that's how every year should feel. And after winning, after coming out of a competition like that, it's got that sort of feel to it -- that it's a new year, and we all have a clean slate and we're not looking back at the past. I'm not looking over my shoulder. I'm just going to take advantage of the opportunity ahead, and it feels good."
Chicago traded for Foles -- the MVP of Super Bowl LII while with the Philadelphia Eagles -- at the onset of the new league year. The Bears later declined Trubisky's fifth-year option -- Chicago selected Trubisky second overall in the 2017 NFL draft, ahead of Patrick Mahomes and Deshaun Watson -- and announced an open quarterback competition in training camp.
"I felt good out there," Foles said on Sunday. "Was I myself, like where I want to be? No, I wasn't, but that's not based on footwork [mechanics] or anything else; that's based on the other circumstances that are out of your control, where you are moving your family, you're with a new offense and you're with new players. You really get a great time in the spring to go through OTA practices and get those cobwebs out. That's not an excuse; that's just a reality of it all."
Foles begins the regular season second on the Bears' quarterback depth chart. Veteran quarterback Tyler Bray was signed to Chicago's practice squad on Sunday.