Cubs infielder Nico Hoerner left the first game of his rehab assignment Sunday in the middle of an at-bat.
The Cubs said Hoerner felt tension along his right oblique after swinging through a 1-0 pitch in Sunday's South Bend Cubs game. He didn't want to push it and played it safe, leaving the game.
Hoerner will be reassessed Monday.
Hoerner, 24, has been on the 10-day injured list since July 29 with a right oblique strain. He sustained the injury a day prior on a check swing in a game against the Reds.
Hoerner was expected to continue his rehab assignment with Triple-A Iowa this coming week.
Before Sunday's game against the Royals, Cubs manager David Ross said Hoerner would take over as the starting shortstop once he returns from the injured list.
Hoerner started at short for South Bend, batting second, before exiting.