Posada was batting in the top half of the frame against Baltimore right-hander Jeremy Guthrie when he was drilled with the pitch.
Posada stayed in the game after being inspected at first base by assistant athletic trainer Steve Donohue. He scored on Nick Swisher's two-run triple, but was replaced by backup catcher Francisco Cervelli between innings.
"When I scored, I hit third base and going home I really couldn't feel the leg," Posada said. "It was kind of numb."
Girardi expressed frustration with Guthrie, who also plunked Mark Teixeira on the right elbow in a March 29 Spring Training game.
"I really don't think you're trying to lead off an inning by hitting a guy," Girardi said. "I don't think he's doing it on purpose, but he hits a lot of people. It's frustrating for us.
"We know he's going to pitch inside and I don't have a problem with pitching inside. But what do you expect, me to be happy our guys are getting plunked? I'm frustrated by it. I wish he had better command."
Guthrie said after the game: "I'm apologetic for the fact that if he were to miss some time. That would be a shame that it was a pitch that got away from me that had that result."
Posada said he would treat the knee with ice to reduce the swelling, and was not pointing fingers at Guthrie for the plunking.
"When you get hit, you always get mad," Posada said. "I don't think he was trying to do anything. It looked like the ball got away from him."
Girardi said that he would speak to general manager Brian Cashman about the possibility of calling up a catcher -- perhaps Jesus Montero or Chad Moeller -- from Triple-A Scranton/Wilkes-Barre in time for Thursday's game.
"We'll have to see how [Posada] is when he gets here," Girardi said.