Posada was scratched from the starting lineup for Wednesday night's game against Boston, and X-rays revealed a nasal fracture. Posada was to remain in the hospital overnight and was expected to be released Thursday morning.
Yankees manager Joe Torre said Posada had minor swelling above his left eye and was bleeding from the nose but his eye socket was fine.
"It's probably going to be about five days [before he can play]," Torre said. "He can work out."
Posada was playing catch with backup catcher Kelly Stinnett when he was hit by the ball.
In other news, Yankees center fielder Johnny Damon, slowed by left shoulder tendinitis, was set to play the field for the first time Thursday since being hurt during the World Baseball Classic, but Torre moved it back to Saturday.
"Just better to be careful," Damon said.
Torre considered letting Damon play center Wednesday.
"I was almost tempted on what he said coming in here before the game that he's ready to go," Torre said. "We'd rather have him go out there and throw one more day."
Damon was the designated hitter on Wednesday against his former team. He left the Red Sox for a $52 million, four-year deal with the Yankees during the offseason.