The injury occurred Wednesday, on the second day of practice.
The redshirt senior started five games last season after retaking the job from Everett Golson, but he was competing with redshirt freshman Deondre Francois for the 2016 starting job.
In the Chick Fil-A Peach Bowl on New Year's Eve, Maguire fractured his left ankle in the first quarter. He returned to the game, but the injury required surgery. He missed all of spring practice recovering. He was participating in summer workouts and preseason practices.
In 2014, he started in place of Jameis Winston for the No. 1 Seminoles against Clemson and helped lead an overtime win.
As a junior, he threw for 1,520 yards, 11 touchdowns and six interceptions.
Francois redshirted last season as a true freshman. He was No. 42 in the 2015 ESPN 300, and he was the third-ranked dual-threat quarterback. He spent his senior season at the IMG Academy.
He has never thrown a pass in a college game. He received many of the first-team repetitions in practice this spring and at Saturday's practice with Maguire out.
When camp opened Tuesday, Fisher said Francois was able to accurately break down why a play succeeded or failed, which is something the seventh-year Florida State coach emphasizes. He said he isn't naming Francois the starter yet.
"[If] they can't answer why, then we're all in trouble. When they make a mistake, if they understand why they made it, then they can fix it if they make a good play," Fisher said. "Every position we evaluate, we ask them, 'Why did you do it?' They have to tell me why and they've got to understand.
"That's why I feel like he's legitimately in the competition to be the guy, because he understands the whys and the why-nots of the game."