CHICAGO -- Chicago Cubs right-hander Marcus Stroman is dealing with right rib discomfort, delaying his return from the injured list.

Stroman was placed on the 15-day IL on Aug. 2 with right hip inflammation. The All-Star was slated to start Wednesday against the White Sox, but manager David Ross said Stroman had some discomfort in the rib area after throwing a bullpen session Sunday in Toronto.

"Concern is just ... we've got to wait and see," Ross said Tuesday.

The Cubs said Stroman, 32, was getting checked out for the issue, and there was no further update. Javier Assad "more than likely" will start the series finale against the South Siders, Ross said.

Stroman, who can become a free agent after the season, had a 2.28 ERA after his first 16 starts this year. But he had struggled before his IL stint, going 1-4 with a 9.00 ERA in his last nine games.

The Cubs are in the mix in both the NL Central and wild-card standings. They are looking for their first playoff appearance since 2020.

Assad pitched seven sparkling innings in a 6-2 victory at Toronto on Friday.

"I think the one positive to all this is Javier Assad's throwing the ball really well," Ross said.

The Cubs also made a minor move before Tuesday's game against the White Sox, recalling right-hander Michael Rucker from Triple-A Iowa. Right-hander Caleb Kilian was sent down.

Rucker, 29, pitched three innings of one-run ball in a start for Iowa on Friday. He is 2-1 with a 5.03 ERA in 34 appearances for the Cubs this year.