Brewers manager Craig Counsell made that announcement Sunday. Counsell also said the Brewers have re-signed reliever Brad Boxberger to a minor league contract and first baseman Daniel Vogelbach has made the team.
Counsell said Topa threw just three pitches in a simulated game Saturday before leaving with elbow discomfort. The Brewers are awaiting results on an MRI that Topa underwent Saturday night.
"It's an injured list to start the season, for sure," Counsell said from the Brewers' spring training site in Phoenix.
Topa, 30, made his major league debut last season and went 0-1 with a 2.35 ERA and 12 strikeouts in 7⅔ innings. He threw two scoreless innings in the Brewers' first-round playoff series loss to the eventual World Series champions, the Los Angeles Dodgers.
His injury reduces the depth of a bullpen that figures to be one of Milwaukee's strengths, thanks in part to two-time All-Star Josh Hader and 2020 NL Rookie of the Year Devin Williams. Counsell noted that relievers Drew Rasmussen, J.P. Feyereisen and Ángel Perdomo have enjoyed productive springs.
"Obviously Justin had a possibility of pitching some big innings for us, so it's a loss," Counsell said. "Guys fill in around losses always, but these are losses that start to chip away at your depth, for sure. Justin pitched some big innings for us at the end of the year last year, and I think he was capable of doing that [again]. Hopefully we get decent news, and he can get back to that."
One option to solve any depth issues is Boxberger, who rejoined the organization after getting released on Friday. Counsell said Boxberger, a former closer for Tampa Bay and Arizona, would be on the Brewers' taxi squad for their first trip rather than going to their alternate site.
Reserve outfielder Derek Fisher also is expected to start the season on the injured list. Fisher has been dealing with a hamstring issue this spring.
The Brewers removed the suspense from one of their top remaining roster issues Sunday by announcing Vogelbach had made the team. Vogelbach's status had been uncertain because the NL isn't using a designated hitter this season.
Vogelbach primarily has been a DH throughout his career. Vogelbach bounced back from a prolonged slump by batting .328 with four homers, 12 RBIs and a .418 on-base percentage in 19 games after the Brewers acquired him late last season.
Counsell said the Brewers will use Vogelbach mainly as a pinch-hitter.
"It's going to be trying to get him a big at-bat every night," Counsell said. "We think he's well-suited for it. We think he can be impactful doing it, putting him in a big spot during a game and having a big at-bat. We think we're covered enough at other positions to be able to do that."
Bradley had been dealing with a sore wrist after having a cyst removed during the offseason. Cain opted out of last season after playing just five games and was battling a quadriceps issue this spring.
They each went 2 of 3 and combined to score five runs in an exhibition game Saturday against the Kansas City Royals. Cain homered in the game.
"I think they'll be ready," Counsell said. "I think in Lorenzo's case, we're going to have to be careful early in the season just because of a leg injury, not playing last year, the weather we may face. We will be careful with Lorenzo specifically, I think, early in the season."