Harrison experienced some shoulder stiffness before Monday night's 11-1 loss to the New York Yankees, then felt some pain during a postgame workout. Harrison gave up seven runs and a career-high 11 hits in five innings against the Yankees.
Rangers general manager Jon Daniels said before Wednesday night's game against New York that it was premature to assume Harrison was headed for the 15-day disabled list.
"It's a possibility but it's too early to tell yet," Daniels said.
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Harrison, with a large bag of ice taped to his left shoulder, said he wanted to "play it safe, let it heal up, and get back after it."
The Rangers will move right-hander Brandon McCarthy, who was scheduled to start the first game of Friday's doubleheader against Oakland, back to Saturday.
Right-hander Scott Feldman will start the second game of the doubleheader, as previously scheduled, but manager Ron Washington was undecided on his starter for Game 1. The Rangers probably will call up a pitcher from the minors, which could lead to Harrison going on the DL.
Right-hander Vicente Padilla will have a lengthy bullpen session Friday as he rehabs from right shoulder bursitis. If that goes well, he could be activated from the 15-day disabled list and return to the rotation during next week's road trip against the Yankees and Boston Red Sox.