It’s a rough day for those competing to be Josh Rosen’s backup at UCLA

It was not the best of times for UCLA’s backup quarterbacks toward the end of practice Thursday.

Devon Modster’s third-down pass was knocked away in the end zone and fell incomplete. Matt Lynch fired a pass over the middle that was intercepted. Austin Burton’s fourth-down pass was dropped.

Back onto the field trotted the backups for one more try. It was more of the same.

Modster’s third-down pass was completed short of a first down. Lynch had another pass intercepted. Burton pumped his arm on third down, searching for a receiver before whistles were blown to signal that the play had taken too long to develop and would be considered a sack.

There was no discernible movement in the battle to become quarterback Josh Rosen’s primary backup. That fight already appears to have been won by Modster, who has been running the second-team offense for the duration of training camp.

“Modster’s right now doing a good job with the twos,” offensive coordinator Jedd Fisch said this week.

Comparisons between the quarterbacks can be tricky because Fisch said the first- and second-team players are running the same offense in practice while the third- and fourth-teamers are focusing more on basics.

“So it’s tough to completely look at, does one guy move the ball or run the ball better?” Fisch said.

Whoever becomes the No. 2 quarterback will be the insurance policy that UCLA hopes to need only at the end of blowout victories.

The backup became central to the team’s fortunes last season when Rosen suffered a season-ending shoulder injury in October against Arizona State. Fifth-year senior Mike Fafaul stepped into a difficult situation, but his play was indicative of a former walk-on with limited experience. He completed 52.2% of his passes for 12 touchdowns with 11 interceptions and the Bruins lost five of their last six games.

UCLA appears to have a few options this season should Rosen go down. The 6-foot-2, 210-pound Modster, a redshirt freshman, combines a strong arm with a soft touch and has progressed in a variety of areas since spring practice.

“His knowledge is improved dramatically, he’s a faster decision-maker, he’s getting away from center quicker, he’s aiming to be more accurate with his throws, he’s put more trajectory on the ball,” Fisch said.

Modster acknowledged some early struggles with taking snaps under center after operating exclusively out of the shotgun formation while at Rancho Santa Margarita Tesoro High but said “I’ve gotten used to it.” He’s also continued to learn new hand signals and become more vocal as he masters Fisch’s offense.

“We’re asking him to really push himself out of his comfort zone, be louder, be more demonstrative, be more animated,” Fisch said, “but that comes with confidence and you have to kind of have that feel like, ‘I really know this.’ It’s one thing to ask him to call a simple play with confidence, it’s another thing to ask him to call a longer play with more syllables with confidence and that’s kind of right now what we’re trying to get done with him so he gets more and more comfortable.”

The baby-faced Burton, a true freshman from West Orange High in Winter Garden, Fla., accelerated his understanding of the offense by enrolling early at UCLA and attending spring practice, and also with lengthy sessions in film study. He’s put more zip on his passes during training camp but overdid it a bit Thursday, repeatedly sailing the ball over the head of his receivers.

Lynch, a redshirt freshman, has an experience edge over Burton but has had several passes intercepted while splitting time running the third-team offense.

Quick hits

UCLA coach Jim Mora called his recent contention to Yahoo Sports that Rosen would not leave for the NFL after this season “pure speculation. I mean, I have no idea. … I was answering a question in a joking way with the guy, so my concern right now is just making sure we play well as a team in 2017 and then we’ll see what happens after that.” … Tight end Austin Roberts, who had been sidelined for several days with an undisclosed injury, participated in practice while wearing a yellow non-contact jersey.

ben.bolch@latimes.com

Twitter: @latbbolch