The right-hander was roughed up over those two innings, allowing four runs on six hits, including homers by B.J. Upton and Jose Molina.
The Rangers would go on to lose to the Rays, 6-0.
Oswalt felt the discomfort in the first inning on a slider to Jeff Keppinger, Tampa Bay's second batter.
"It just kind of tightened up, my forearm did," Oswalt said. "[In] a couple of days, it should be all right."
This was Oswalt's first start since Aug. 23, and because he isn't in the rotation regularly, he feels he won't miss much time.
"If I had to keep starting, I'd probably be concerned a little more," he said. "Going out, throwing here and there, it shouldn't be too bad."
Lefty Martin Perez came on in relief with Texas trailing, 4-0, and surrendered two more runs -- both on solo homers by Upton -- over five innings.
Greg Zeck is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.