The news comes in light of a tightly contested starting quarterback battle this preseason between Brennan, Arizona State senior transfer Jayden Daniels and redshirt freshman Garrett Nussmeier. There's also freshman Walker Howard, although the expectation is he will redshirt this season.
But with Brennan out of the picture, where does LSU's quarterback battle stand under first-year coach Brian Kelly? Here's a look at who may have the edge after 10 preseason practices.
Why Jayden Daniels should be the starting quarterback
Daniels has two advantages over Nussmeier: starting experience and mobility.
Daniels started 29 games at Arizona State and threw for 6,024 yards with 45 total touchdowns. Nussmeier has never started a collegiate game.
That prior exposure matters under Kelly, who tends to play quarterbacks with experience.
At Notre Dame, Kelly's last starter was Wisconsin transfer Jack Coan, who had started 18 games with the Badgers before winning the job. The quarterback prior to Coan, Ian Book, didn't become the Fighting Irish's starter until the fourth game of his junior season.
Daniels is also a very good athlete who is not afraid to tuck the ball and run. He rushed for 1,288 yards and 13 touchdowns during his time at Arizona State and last season ran for 710 yards, second most on the team.
His running ability is appealing to offensive coordinator Mike Denbrock, a staunch believer in the value of mobile quarterbacks in his offenses.
"I think mobility is a key anytime at the quarterback position, especially when you're playing against some of the dynamic athletes that there is upfront in SEC defenses," Denbrock said last week.
During Thursday's practice, viewed by the media in its entirety, Daniels took the vast majority of the snaps with the first-team offense.
He looked comfortable and confident making throws and maneuvering within a tight pocket. During the final period of practice against LSU's first-team defense, Daniels was 6-of-8 passing.
MYLES BRENNAN LEAVES LSU FOOTBALL: LSU football QB Myles Brennan leaves program, retires from football
LSU FOOTBALL MISSES OUT ON AP POLL: LSU football misses out on the top 25 of the preseason AP poll for the first time since 2000
JORDAN TOLES ENTERS TRANSFER PORTAL: LSU football safety Jordan Toles enters the transfer portal
Why Garrett Nussmeier should be the starting quarterback
Of the three quarterbacks this spring, Nussmeier had the strongest performance at the spring game. And in the weeks leading up to the game, he emerged as a starter despite being viewed as a backup heading into the spring.
Like Daniels, Nussmeier is a good athlete, even though he utilizes his skills differently. But unlike Daniels, Nussmeier does not have as many questions surrounding his accuracy.
LSU's coaching staff has been trying to adjust Daniels' footwork since the spring to help improve his accuracy. He's been more accurate as a passer during preseason practices, but the question still remains after he only threw 10 touchdown passes last season.
Daniels may have taken the lion's share of first-team snaps on Thursday but Nussmeier was limited, as he was dealing with a Grade 1 ankle sprain, which is not considered severe. Kelly told the media after practice that Nussmeier would have had first-team reps if he was healthy.
Daniels has some accuracy concerns, but his starting experience and superior mobility give him the edge in LSU's quarterback battle through 10 practices.
Koki Riley covers LSU sports for The Daily Advertiser and the USA TODAY Sports South Region. Email him at firstname.lastname@example.org and follow him on Twitter at @KokiRiley.
This article originally appeared on Lafayette Daily Advertiser: Making sense of LSU football QB battle after Myles Brennan retirement