Millwood said he felt the muscle tighten up while striking out John Buck for the second out in the second inning. He faced one more batter and retired Alex Gordon on a grounder to first to end the inning.
When the Rangers came to bat in the third, Dustin Nippert started quickly warming up in the bullpen. When the inning was over, Millwood went back to the mound and started warming up, hoping to stay in the game.
"I just couldn't get out on my front leg," Millwood said.
Millwood is unsure about the extent of the injury, but is hopeful that he will not have to miss a start.
"I'm sure we'll probably know more tomorrow," Millwood said in the Rangers clubhouse after the game. "It's starting to feel better. Hopefully it's just something that tightened up and will go away quickly."
Millwood, who leads the Rangers with 138 innings this year, is the only member of the Opening Day rotation that has not been on the disabled list this year. Four others have: Vicente Padilla, Matt Harrison, Brandon McCarthy and Kris Benson. Scott Feldman has taken over for Benson, who is at Triple-A Oklahoma.
Harrison is out for the year and McCarthy is out until September. They have been replaced by Derek Holland and Tommy Hunter.
T.R. Sullivan is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.