Mientkiewicz was struck in the head by the left knee of Boston's Mike Lowell in a collision at first base, which came as Lowell was beating out a potential double-play grounder against New York's Scott Proctor.
The baseball trickled away and two runs scored as Mientkiewicz lay motionless on the infield dirt, at which time play was halted. Tests at Massachusetts General Hospital revealed a mild concussion, cervical sprain and a fractured scaphoid bone in Mientkiewicz's right wrist.
The first baseman will be kept overnight for observation and will be replaced on the Yankees' roster before Sunday's game against the Red Sox.
"Right now, that's really all that's on my mind," said Proctor, who was the losing pitcher in New York's 11-6 defeat. "You get a guy, a teammate like that, who's going out there busting his [rear] every day, and lose him. To see him take such a hard hit, we're hoping and praying that he's going to come out OK. He's a gamer, he's a big part of this team, and we want him back."
Lowell hit a bases-loaded ground ball to second baseman Robinson Cano, who fed the play to shortstop Derek Jeter on an awkward throw, forcing out Kevin Youkilis at second base. Appearing to have issues gripping the ball, Jeter relayed a one-hop bouncer to Mientkiewicz.
Mientkiewicz, a former Gold Glover, went into a split in an attempt to corral the throw but was clipped by Lowell, who barreled down the line and went to second base on the throwing error charged to Jeter.
"It's frightening," Yankees manager Joe Torre said. "[Mientkiewicz was] trying to catch the ball, and it was an ugly play to start with. You could see Dougie sort of backing in to try and get a good hop to catch it, and Mike ran him over. I don't think [Lowell] did anything wrong, though."
"That was almost inevitable," Red Sox manager Terry Francona said. "It was just two bodies in the way of each other. That's tough. That makes you nervous, regardless of what team you're with."
Lowell expressed concern over Mientkiewicz's state.
"Doug's a guy I hang out with sometimes in the offseason," Lowell said. "I know him from growing up in Florida. I just hope he's OK."
Mientkiewicz lay motionless on the ground for several moments before being attended to by team trainer Gene Monahan, team physician Dr. Stuart Hershon and Torre, as Mientkiewicz sat on the infield with dirt caked in his hair and on his face.
Torre said Monahan checked Mientkiewicz's extremities and sensations of feeling, which the manager relayed all checked out. Torre said Mientkiewicz had a lump over his left eye.
"Geno was conversing with him," Torre said. "As time went by, he seemed to be a little more [aware]. He never lost consciousness."
Torre added that Mientkiewicz alertly overheard a conversation discussing a possible stretcher or medical cart, which Mientkiewicz interrupted and said he wouldn't need. Regardless, the Red Sox summoned the cart to escort Mientkiewicz, who won a World Series ring in 2004 with Boston, off the field.
As Josh Phelps took over at first base, the crowd of 36,924 gave Mientkiewicz an ovation as he was driven out the left-field gate.
In 50 games for New York this season, Mientkiewicz is batting .224 with four home runs and 16 RBIs.
Bryan Hoch is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.