"All I know is that he said he didn't think his ankle was working right," Girardi said.
Bird was placed on the disabled list on May 2 with a right ankle bruise after a rough start to the season (.100/.250/.200), which came after an excellent performance in Spring Training. Girardi said his strong spring added to the frustration.
"It's something most teams end up dealing with, an injury you don't want to have and it could be costly for you," Girardi said. "Fortunately, our offense has been really, really good. But we thought he'd be a big part of our offense, but he's not, and it's frustrating."
When asked if there was a chance the Yankees wouldn't see Bird again this season, he added, "I'm not sure. If it was a stress fracture, you would say, 'Yeah.' But all that's been eliminated. We'll see."
Bird had played in 12 games between Class A Advanced Tampa and Triple-A Scranton/Wilkes-Barre during the rehab assignment, hitting .237/.420/.342 in 50 plate appearances while playing first base.
Before the rehab assignment, Bird had been running the bases and was considered healthy enough for game action.
Jack Baer is a reporter for MLB.com.
Alex Simon is a reporter for MLB.com based in the Bay Area. Follow him on Twitter at @alexsimon99.
This story was not subject to the approval of Major League Baseball or its clubs.