An announcement was made about one hour before the scheduled first pitch of the Yankees' game against the Blue Jays, the second of a three-game series.
"His shoulder's a little stiff, so we just thought [with] the weather and the breeze, we'll just play it safe," manager Joe Girardi said. "It's too early in the year. He's day-to-day."
Jose Molina was inserted in Posada's place, batting eighth and catching starter Mike Mussina. Girardi said that Posada believes the stiffness results from his awkward throw to second base in the fifth inning of New York's season opener on Tuesday, when Marco Scutaro stole on Chien-Ming Wang.
Posada was not sent for tests, though he did receive treatment at Yankee Stadium. Girardi said that Posada would be available to pinch-hit on Wednesday if needed.
"It's nothing I'm concerned about," he said. "I don't expect this to be long."
Posada, 36, hit a career-high .338 with 20 home runs and 90 RBIs in 144 games for the Yankees last season, earning a new four-year, $52.4 million contract after the campaign. He went 0-for-3 with a strikeout on Tuesday against Toronto.
Bryan Hoch is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.