Posada was hit on the right knee by a Jeremy Guthrie pitch in the second inning of New York's 8-3 victory over Baltimore on Wednesday, and the 38-year-old spent most of Thursday in the treatment room with assistant athletic trainer Steve Donohue.
"No question, there's been improvement since last night," Posada said. "There's still some swelling that we need to get out, but if need be, in a pinch, I can play tonight."
Manager Joe Girardi checked with Posada earlier on Thursday, learning that his catcher was mostly sore and that he could bend his knee. For that reason the club was never close to calling up a catcher from the Minor Leagues, and Posada was not sent for tests.
In the event that something did happen to Francisco Cervelli, who is catching A.J. Burnett on Thursday, Girardi would probably go first to Posada rather than to emergency third catcher Ramiro Pena.
"If something were to happen to Cervy, [Posada] could get behind home plate," Girardi said. "Running, he's probably not going to feel great, but that was never one of his strong suits."
Girardi expects Posada to continue improving as the Yankees return home on Friday to open a homestand with the White Sox, but he considers Posada "doubtful" to start the first game of that series.
"I think every day he's going to get better," he said. "He may be more than an emergency catcher tomorrow. I think we're going to be fine."