CHICAGO -- The Cubs were expecting to have Marcus Stroman off the injured list and back on the mound at Wrigley Field on Wednesday night. That plan fell apart when the All-Star pitcher reported discomfort in his right side as the team left Toronto this past weekend.

Now, the Cubs are facing an uncertain timetable for Stroman, who was diagnosed with a right rib cage cartilage fracture. Righty Javier Assad moved into Stroman’s rotation slot on Wednesday against the White Sox, and the North Siders will enter wait-and-see mode when it comes to the veteran’s latest comeback bid.

Prior to Wednesday’s 4-3 walk-off win over the White Sox, Cubs president of baseball operations Jed Hoyer was asked if Stroman’s season could be at risk of ending.

“I have no idea. That’s the honest truth,” Hoyer said. “We don’t really know at this point. Obviously, we’ll give him a real rest period and see how he feels. But I mean, again, it’s not a real common pitching injury. I’ve never seen that before.”

Stroman has been on the IL since Aug. 1 due to right hip inflammation, but the righty worked through three bullpen sessions on the team’s recent road trip through New York and Toronto with encouraging results. He completed a 50-pitch mound workout on Friday and another light session on Sunday, putting him on target to start Wednesday.

On Sunday morning in Toronto, Stroman told reporters that he was “progressing right where I feel like I should be” and expressed excitement to come off the IL. By the end of the day, he informed the team that he was experiencing pain in his side, leading to an MRI exam on Monday’s team off-day.

Both Hoyer and manager David Ross said it was not clear how the rib setback occurred. Stroman was not available for comment prior to Wednesday’s game against the White Sox.

“Disappointed, obviously,” Ross said. “We’ve just got to wait and see just how long and how much pain he’s actually in. Getting him back, we’re going to have to rely on him being symptom free. So, how long that takes, nobody really knows. So, it’s just kind of up in the air.”

Against the White Sox, Assad pieced together his second straight quality start, allowing three runs (two earned) over six innings for the North Siders. A lack of run support led to a no-decision for the righty, who has turned in a 1.38 ERA over his last 12 appearances for the Cubs.

“We’re a better team when [Stroman’s] himself and out there for us,” Cubs second baseman Nico Hoerner said. “But we’ve had pretty admirable work from our staff filling in for him. And we’ll continue to do that.

“[Assad] is just a guy who I feel like is not really going to be showing up too different one way or the other – and I mean that in the best way. He’s been comfortable and confident with everything he’s been handed.”

Assad is likely to remain in the rotation for the time being, and the Cubs have solid depth at the top of the staff between Justin Steele, Jameson Taillon and Kyle Hendricks. Behind that group, lefty Drew Smyly and righty Hayden Wesneski offer a pair of options.

Both Smyly and Wesneski were in the Opening Day rotation, but recent struggles led to respective moves to the bullpen. As things stand, Ross said he is leaning toward giving Smyly the No. 5 rotation slot, which will come next on Tuesday in Detroit.

“We’ve got some depth,” Ross said. “Sometimes that gets tested and other guys have got to step up when you want to go to the playoffs. Every team deals with little things that pop up like this. We’ve got a lot of professionals in that room. A lot of good big leaguers. We’re going to be fine.”

The 32-year-old Stroman spun a 2.28 ERA in his first 16 starts, which featured a 7-0 run with a 1.29 ERA in seven starts leading up to the London Series at the end of June. During that June 25 outing, a blister flared on the All-Star’s pitching hand, leading to an early exit at the front end of a rough stretch (9.00 ERA in seven starts).

The Cubs were looking forward to getting the first-half version of Stroman back for the stretch run, but others have been picking up the slack. Going back to June 25, the Cubs have a 24-19 record (entering Wednesday) and have climbed back into the National League Central and Wild Card races.

“Certainly, we were hopeful to get him back tonight,” Hoyer said, “and [for him to] start pitching the way he was in April and May. Obviously, that didn’t happen. But, we’ve been playing great baseball for quite a while. I think this time of year, you just sort of roll with things like that and move forward. We have depth.

“We’ve been obviously using that depth for a while and performing, so we have to keep doing that. This time of year, you don’t stop and wallow too much. You say, ‘It’s too bad. We’d love to have him pitching for us,’ but he’s not. And we’ll keep playing well.”