The Royals ran wildly on Posada, stealing four bases -- three by speedster Joey Gathright -- and Yankees manager Joe Girardi acknowledged that keeping Posada behind the plate would now be a weakness.
"It's not sore," Girardi said. "The ball is just not coming out of his hand the way you want it to. His footwork is fine and everything. We'll have to sit down and talk about that tonight and evaluate what we're going to do."
Posada, 36, said that he expects to have an MRI focused on the back of his shoulder on Wednesday in Kansas City. Neither Girardi nor Posada would rule out a stint on the disabled list.
"We need to know what's going on," Posada said. "It's really early in the season, and we've got to be smart about it now. I don't want to be back there when I'm not helping the team out."
Posada, batting .176 (3-for-17) this season, said that the stiffness does not affect him while he is hitting, but he is unable to get any zip on his throws -- either to the bases or back to the mound.
The ailment has affected Posada for brief periods in the past, most recently during Spring Training, but unlike previous times, Posada has been unable to recover from an awkward throw made against the Blue Jays on April 1.
"It doesn't take that long to heal -- that's what I'm worried about," Posada said. "It usually takes four or five days and you're back at it. I had it during the spring, and now I have it a little bit now. I was throwing good down in Florida, so we'll see."
Posada was replaced by backup catcher Jose Molina for the top of the seventh inning. Molina, batting .286 (4-for-14), would be the interim understudy if Posada does not land on the disabled list.
While Posada could stay active as a designated hitter, Girardi said that he would prefer not to take the bat away from Hideki Matsui, who is hitting .357 through eight games. Girardi said he planned to speak with general manager Brian Cashman and the Yankees' training staff on Tuesday evening regarding the injury.
Journeyman Chad Moeller is New York's Triple-A catcher and could see a callup in the event of a DL assignment. Posada said he was hopeful that he would be able to avoid one, but his tone did not seem optimistic.
"I can't be out there if I'm not 100 percent," Posada said. "A catcher is like a pitcher -- you need your arm. If you were an outfielder or a shortstop, you could play through it. We throw as much as the pitcher does."