Marcus Stroman won’t start Wednesday for the Chicago Cubs — and his return from the IL is uncertain — after suffering rib discomfort

Everything was lined up for Marcus Stroman’s return from the injured list to start Wednesday’s City Series finale for the Chicago Cubs.

Instead of coming off the 15-day IL to take on the White Sox, it’s unclear when Stroman will make his next start due to right rib discomfort. The 32-year-old right-hander hasn’t pitched since July 31 because of right hip inflammation.

A high-intensity, 50-pitch bullpen session went well Friday in Toronto, and a normal between-starts bullpen Sunday represented the last step before being activated Wednesday.

However, Stroman experienced the rib discomfort later Sunday after throwing his pregame bullpen. He was getting checked out Tuesday.

Right-hander Javier Assad “more than likely” will start Wednesday versus the White Sox, manager David Ross said before Tuesday’s series opener at Wrigley Field which the Cubs lost 5-3.

Assad is coming off a stellar start Friday in Toronto, where he held the Blue Jays to one run and four hits in a career-high seven innings in a 6-2 Cubs victory.

“The one positive to this is Javier Assad is throwing the ball really well,” Ross said. “That’s kind of how I’m looking at it. … We’ve got to keep pitching well. If (Stroman is) out longer or pushed back a couple days, the concern is we’ve just got to wait and see.”

The Cubs (61-57 entering Tuesday) are entering a critical stretch of their schedule.

They have a chance to take advantage of four consecutive series against teams under .500 — at home against the Sox and Kansas City Royals and road series against the Detroit Tigers and Pittsburgh Pirates — before a three-game set at Wrigley against the first-place Milwaukee Brewers (Aug. 28-30). Then comes four games in Cincinnati (Sept. 1-3) against the Reds, who entered Tuesday tied with the Cubs sitting 3 1/2 games back.

The Cubs believe that Stroman’s struggles leading up to his IL stint stemmed from his mechanics getting out of whack and that a healthy version will pitch more like he did during his All-Star first half. He posted a 9.00 ERA over his last seven starts.

“Stro’s been at the heart of a lot of our success this year,” second baseman Nico Hoerner said Tuesday, “and we definitely would not be in the position to add players at the deadline without everything he did in the first half to get us to that point. So obviously a huge factor for us.”

Stroman was encouraged by where he was ahead of Sunday’s bullpen, focused on fine-tuning the little things.

“Just getting into where I need to be to where I don’t have to think, honestly, on the mound,” Stroman said Saturday. “I’m best when I’m not thinking at all, not having to worry about this, this, this and this.

“Getting to that point — which has a physical component and then the physical kind of leads to the mental component — it’s getting those few little checked boxes, little things I need to zone in and click it in, which I should be there by the time I pitch.”

()