The Cubs placed reliever Manny Rodríguez on the 10-day injured list (shoulder inflammation) prior to Friday's series opener against the Brewers.
In a corresponding move, the club reinstated lefty Adam Morgan from the bereavement list.
With only two weeks remaining, Rodríguez' season is likely over. He worked his way back this season from a right biceps strain sustained in 2020, a factor as the Cubs proceed with caution.
"Probably err on the side of making sure that everything's fine," manager David Ross said.
"It makes a lot of sense to take care of him because of his history," Ross added, "and then this extended season that he's gone through and the workload he's handled this year."
Rodríguez made his big-league debut at the end of July following a promotion from Triple-A Iowa. The right-hander holds a 6.11 ERA in 20 appearances, striking out 16 with 12 walks in 17 2/3 innings.
He's allowed 11 runs (eight earned) in his last four appearances.
Rodríguez, who wields a fastball that touches 100 mph, is part of the Cubs' bullpen plans heading into 2022. The 25-year-old represented the Cubs at the Futures Game weeks before his promotion.
"I thought he's done phenomenal working his way back in spring training," Ross said. "A little bit of command issues to getting locked in and being able to help us out in a lot of moments. Closed out a game for us.
"I've seen some really good signs of a guy that can definitely help us out."
Morgan went on the bereavement list last Friday. He's been one of the Cubs' most reliable relievers over the last month.
In 15 appearances since August, the veteran holds a 2.25 ERA, allowing three earned runs in 12 innings.
Contributing from Milwaukee: Gordon Wittenmyer