Jason Heyward landed on the 10-day injured list Thursday with right knee inflammation, the Cubs announced.
"It's been bothering him for quite some time," manager David Ross said. "The training staff is just trying to do the best they can to get that thing fixed."
It's unclear how serious the injury is. Heyward has not undergone an MRI yet, according to Ross, but that seems like a likely next step.
The veteran outfielder hasn't played since Friday's series opener against the Cardinals. Ross described him as "banged up" before Wednesday's win over the Reds.
Heyward has struggled at the plate this season, the seventh of an eight-year contract he signed before 2016. He's hitting .204/.278/.277 in 48 games.
After moving to center field the season to accommodate the arrival of Seiya Suzuki, he's exclusively played right since Suzuki landed on the IL in late May.
Suzuki began a rehab assignment Thursday with Triple-A Iowa.
In a corresponding move, the Cubs promoted outfielder Narciso Crook from Iowa. Crook, 26, would be making his big-league debut the first time he takes the field for the Cubs.
To make room for Crook on the 40-man roster, Michael Hermosillo (left quad strain) was transferred to the 60-day IL.