Hamels said he felt the groin begin to bother him in the fourth inning of Friday's start against the Mariners. He tried to pitch through it but couldn't hold a 3-1 lead. Instead, he allowed a run in the fifth and two in the sixth inning of a 4-3 loss.
"That's what I don't want to happen again," Hamels said.
"It's not something you want to happen, especially when I have been feeling good all year. I understand the situation, where the team is at and what I want to do here. I want to play a good part in this, but if I'm not able to do that close to 100 percent, I don't want to jeopardize the team or the bullpen. So we'll play it out, be smart and miss a game."
The Rangers, whose acquisition of Hamels from the Phillies was finalized on July 31, said Hamels had a similar issue with his left hamstring in June and missed only one start. He will likely throw a bullpen session on Saturday, and that could determine when he'll pitch again.
"My level of concern sits with him," Rangers manager Jeff Banister said. "He doesn't seem too concerned."
Gonzalez is scheduled to pitch Wednesday for Round Rock and Ranaudo is scheduled for Thursday. Both have pitched with the Rangers this season.
Gonzalez was 2-4 with a 3.74 ERA in seven starts for the Rangers, while Ranaudo was 0-1 with a 7.63 ERA in two starts and two relief appearances.
T.R. Sullivan is a reporter for MLB.com. Read his blog, Postcards from Elysian Fields, follow him on Twitter @Sullivan_Ranger and listen to his podcast. This story was not subject to the approval of Major League Baseball or its clubs.