As reasons for the switch, Flores cited Tagovailoa's improvement in practice and readiness as well as an overall feeling within the organization that it was time.
"We feel like through practice meetings and walk-throughs that he's ready, and that's how we're going moving forward," Flores said. "[Ryan Fitzpatrick] has done a great job. He's been productive. His leadership has been great. It's not an easy decision for me or us as an organization, but we felt like for the team now and moving forward, this is the move we needed to make."
The Dolphins discussed the move Monday night with coaches and front-office personnel before making the decision to start Tagovailoa. Flores said being on the bye week helped and that Tua making his NFL debut in garbage time against the New York Jets on Sunday was another item to cross off his to-do list.
Ultimately, however, Flores said the feeling was that Tagovailoa had shown enough behind the scenes to be given the keys to the offense.
"We've seen a lot of improvement from Tua," Flores said. "He's gotten more comfortable -- accuracy, decision-making have been good in practice. But again, practice is very different than games. We're going to do everything we can from a meeting, walk-through, practice standpoint to get him ready to play in a game. Obviously we're comfortable and confident that he'll be able to be competitive in those games when that time comes. There's a lot that leads into that."
Multiple Dolphins players told ESPN that Tagovailoa has been impressive in practice in recent weeks, with his improvement since training camp becoming more noticeable.
Tagovailoa declared himself 100% healthy after his NFL debut Sunday, and he thought it was important to show mobility as he did on his first rollout pass.
"He's practiced well," Flores said. "He works hard in meetings and walk-throughs. He's got a good rapport with his teammates. Every week we're going out to win, so, yeah, I'm confident that if we prepare the way he's been preparing, then I'm confident that we'll go out there and have good results. But it's not a one-man game."
Flores revealed that he didn't get a chance to address the team about the QB change before ESPN and other outlets reported Tuesday that Tagovailoa was the new lead quarterback.
"One thing on this situation that's unfortunate is that I didn't get a chance to address the team before this was out in the media. That's not the way I or we want to do business," Flores said. "That's unfortunate. I'm not happy about that at all. So I'll address that to the team and apologize that they had to find out on social media because I don't think that's fair to him."
The other side of the coin is that the move sends Fitzpatrick, a beloved player in the locker room, to the bench. He was playing really well, so the move wasn't about his play as much as it was about Tagovailoa's development.
Fitzpatrick completed 70.1 percent of his passes this season for 1,535 yards -- an average of 278.3 yards per game -- and a 95.0 passer rating. He had 10 touchdowns and seven interceptions.
Fitzpatrick has known for a while this change would come, calling himself Tagovailoa's "placeholder" and promising he would be the rookie quarterback's "biggest cheerleader" when it happened.
"Fitz has been great. He's been great really for the last year and a half," Flores said. "I don't know if anyone has been more instrumental in trying to instill a culture and embody a lot of the things we're looking for from a toughness, competitiveness, team-first [perspective]. He's made an impact on Tua and a lot of other players on this team.
"I have great respect for him, and that made this decision very tough. At the end of the day, we felt like as a team that this was the best thing for the team now and moving forward to go with Tua. Fitz, the way he handles himself and his impact on the team, will still be there. He's been a tremendous asset and leader on this team."