Mills has a low back strain, and Miley is dealing with left elbow inflammation. Simmons is working through right shoulder inflammation.
Mills’ IL stint helped pave the way for right-hander Michael Rucker to make the Opening-Day roster for the first time. Rucker, who turns 28 on April 27, had no record and a 6.99 ERA in 20 appearances for Chicago last summer in his first big league action.
Cubs manager David Ross called the move for Mills more of a precaution.
“Had some low back tightness a couple days before he threw after a bullpen,” Ross said before Chicago’s opener against Milwaukee. “Felt it loosen up a little bit to kind of make his outing, and just got done and it just wasn’t very good.”
The Cubs are hoping Miley can help shore up their rotation after a tough 2021 season. Miley was claimed off waivers on Nov. 5 after he went 12-7 with a 3.37 ERA in 28 starts for the Cincinnati Reds, throwing his first career no-hitter in May.
The slick-fielding Simmons agreed to a one-year, $4 million deal with Chicago last month. The 32-year-old Simmons has won four Gold Gloves, but he batted just .223 with three homers for Minnesota last year.