"Disappointment for Mike, but I think there is some excitement for Mitchell," Bears coach John Fox said of how the quarterbacks handled the change. "It was a decision I thought needed to be made. We had 10 giveaways in the first four weeks of the season -- you can't win football games that way. Not that they were all one guy's fault, but the combination of that leads to us going in a different direction."
The Bears traded up to the No. 2 overall pick in the 2017 draft to select Trubisky, who impressed in the preseason by passing for 364 yards, three touchdowns and zero interceptions for a 106.2 passer rating.
Watson (selected 12th overall) has thrived since becoming the Texans' starting quarterback, nearly orchestrating an upset of the New England Patriots in Week 3 and tying an NFL rookie record with five touchdowns (four passing, one rushing) in Houston's 57-14 Week 4 rout of the Tennessee Titans. Kizer (picked in the second round, No. 52 overall) has struggled, throwing an NFL-worst eight interceptions amid the Browns' 0-4 start.
Trubisky won Chicago's primary backup job in Week 1, beating out veteran Mark Sanchez, who has been inactive the first four weeks and will remain the team's third-string quarterback. But there have been questions about Trubisky's readiness to start at quarterback in the NFL after starting just 13 games at the University of North Carolina.
"He's improved [since the preseason]," Fox said of Trubisky. "He has a better understanding of NFL defenses. He's prepared and watched the opponent and been ready to go in and play. I've seen him grow in recognizing those types of things. Even in practice."
The Bears signed Glennon in free agency, paying a hefty price -- he will earn $16 million guaranteed in 2017.
Glennon also struggled in Week 2 against the Tampa Bay Buccaneers, turning the ball over three times (all in the first half) in Chicago's 29-7 loss.
According to Elias Sports Bureau research, Glennon is the only Bears player in the past 40 seasons to have three turnovers in a half twice within the first four games of a season.
"We just have to put an end [to all the turnovers]," Fox said. "It's not all one person as I told the team, but we're just going to go in a different direction."
The Bears have lost three of four games with Glennon at quarterback.
ESPN's Jeff Dickerson contributed to this report.