That move is normally reserved for COVID-19 related issues.
Stroman, 31, was replaced in Sunday's game by lefty Justin Steele, who was removed at the start of the fifth inning because of thumb soreness.
"Just like gripping the slider, when I was pushing down on the bottom of the ball, it was just sore on the top of my thumb and then it just kind of wasn't getting better,'' Steele said after the loss. "I was trying to stretch it out in between innings and stuff.''
Stroman had just started to find his stride after struggling in his first few appearances with the Cubs after signing a 3-year, $71 million deal with them this past offseason. He threw seven shutout innings against the Milwaukee Brewers in his most recent outing.
Righty Adrian Sampson was called up from Triple-A Iowa to take Stroman's place on the roster. The team also sent down struggling first baseman Frank Schwindel.
The Cubs have lost five in a row.
The Associated Press contributed to this report.