McCullers, who first experienced soreness in the forearm in February, had been throwing bullpen sessions earlier this month. He has yet to pitch in a game this season.
"We have to take it slow with Lance until we know that he's 100 percent. That's where it is right now," Astros manager Dusty Baker told reporters Monday.
It's unclear if the setback will impact the team's estimate of the All-Star break as a landing spot for McCullers' return.
McCullers missed most of last season with a strained forearm sustained during the 2021 postseason. He went 4-2 with a 2.27 ERA in eight regular season starts and 0-1 with a 5.87 ERA in three playoff starts.
McCullers' injury absences include the entire 2019 season after he underwent Tommy John surgery.
In 2021, McCullers went 13-5 with a 3.16 ERA in a career-high 28 starts. Overall, he is 49-32 with a 3.48 ERA in 127 starts and three relief appearances over seven seasons.