Gardner, who hurt his right elbow while diving on Tuesday, was hitting .321 with a pair of stolen bases before going down.
"We thought it was his wrist at first, which came up a little bit sore," Girardi said. "But he landed right [on his elbow] and it got tucked under. The big thing is that we don't have a setback -- that the first time he swings, he doesn't feel it."
Girardi is confident the speedy outfielder will be ready when the 15 days are up, though the Yankees won't rush him back. Andruw Jones (against lefties) and Raul Ibanez (against righties) have split the duties in left field during Gardner's absence.
"My thought is he will [be ready]," Girardi said. "If it takes a few more days, it takes a few more days."