The 35-year-old southpaw pitched the first inning in the Cubs' 6-3 loss to the Cincinnati Reds before exiting the game.
"I felt it on the cutter to [Scooter] Gennett, you know, and kinda felt more like a cramp," Hamels said after the game, according to ESPN's Jesse Rogers. "Because I strained or tore my oblique a couple years ago, and that was completely different. That's the one where you feel like you just got shot. So, I really thought it was more of a cramp.
"Continued to throw the next two pitches, and it still was lingering."
Overall in 2019, the four-time All-Star has posted a 6-3 record in 16 starts with a 2.92 ERA and 1.19 WHIP. In June, per ESPN, Hamels had a 1.00 ERA and a 2-1 record across five starts entering Friday.
The potential loss of Hamels would hurt especially right now as Kyle Hendricks remains on the injured list with right shoulder inflammation.
Depending on what moves the Cubs do make, Maddon could turn to Tyler Chatwood, Mike Montgomery or Adbert Alzolay should Hamels miss extended time. Chatwood has started three games this season, while Alzolay has started once, and Montgomery has not started.
The Cubs are 44-38 and hold a one-game lead over the Milwaukee Bucks for first in the NL Central.