Youkilis took a one-hopper in the eye during warmup tosses to start the fifth, suffering the contusion. A CT scan was negative.
The ball, thrown from third baseman Mike Lowell, skipped past his glove and hit the 2007 American League Gold Glove Award winner in the face.
"His eye was getting puffier pretty much by the second," manager Terry Francona said. "By the time I got out there, he wanted to stay in -- he always wants to stay in -- the way it was swelling and as quickly as it was swelling, there was no way we were going to let him hit. He was leading off, so we got him out, got some ice and we got him checked."
With Sean Casey beginning his three-game suspension resulting from Boston's June 5 fracas with Tampa Bay, Brandon Moss replaced Youkilis at first base. It was Moss' Major League debut at first, as he regularly gets his playing time in the outfield, but he said he regularly takes ground balls in the infield.
Still, Moss wasn't so fortunate in his first outing at first base. In the seventh, he momentarily bobbled a ground ball that allowed the eventual game-winning run to score from third.
"I'm pretty comfortable there despite what I showed tonight," Moss said. "I've been taking ground balls and I've been playing there all year in Triple-A [Pawtucket]. It's just one of those things where I let everything speed up on me a little bit."
As for Lowell, who tossed the ball that skipped up on Youkilis, he said he felt terrible about a toss that is generally a routine occurrence during warmups.
"I probably short-hop him five times in a game in between innings," Lowell said. "It wasn't even a hard throw; I kind of lobbed it."
Lowell thought the ball hit Youkilis in the side of the head at first, but noticed rather quickly there was something wrong with the first baseman's eye.
"He got a pretty big shiner pretty quick," Lowell said. "I'm just glad they said he did all those tests and they say he's going to be fine. Hopefully he'll be back [Tuesday]."
Mark Remme is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.