"It was just the heat of the game," he said through an interpreter. "I was trying to catch the ball, I was trying to put my glove between me and the sun, and I just hit the rail."
Girardi said that he does not expect Mustelier to play until Tuesday.
"I couldn't tell, because his back was to me," shortstop Derek Jeter said. "I didn't know if he saw [the railing] or not, but he didn't see it. That was rough. I think he'll be all right."
Yankees general manager Brian Cashman has said that Mustelier's live bat is giving him a chance to make the roster this spring while the team searches for possible replacements for injured stars Curtis Granderson and Mark Teixeira.
A 28-year-old Cuban refugee who has played mostly outfield in the Yankees' Minor League system, Mustelier has received more of a look at third base of late and said he senses an opportunity is here.
"That's something that you're always thinking, and that's why I'm playing hard, and that's why I will keep playing hard," Mustelier said.
Girardi said that Mustelier has forced the Yankees to notice him during this camp, which could open the door for his promotion later in the year, even if he does not break camp with the club.
"We'd talked about [how Mustelier] was a young man that could hit and had played mostly outfield," Girardi said. "He'd played a little bit of third with the Yankees, but had played third mostly in Cuba. He's looked pretty good over there. He's made the plays. ... He's more athletic than I thought. He runs pretty well. He's made an impression."
Bryan Hoch is a reporter for MLB.com. Follow him on Twitter @bryanhoch and read his MLBlog, Bombers Beat. This story was not subject to the approval of Major League Baseball or its clubs.