Girardi said that Robertson will have a CT scan and a weight-bearing X-ray taken on his sprained right midfoot, an injury that he suffered on Wednesday evening after falling down a flight of stairs while moving boxes at his residence.
Robertson was taken for an X-ray on Thursday morning, which was negative, before having the MRI. Team physician Dr. Christopher Ahmad will review the tests in New York and consult with foot specialist Dr. Justin Greisberg. The Yankees expect to have more information on Friday.
"You hope a lot of times you can clear things up with one test," Girardi said. "I don't want to speculate. I would love to know what's going on exactly, if there's something that we're going to miss him for a substantial period, but I don't know."
Robertson was fitted for a walking boot and will have the use of crutches. Speaking to reporters at George M. Steinbrenner Field on Thursday morning after his MRI, Robertson said that he was taking boxes out for recycling when the mishap occurred.
"I just misjudged one step and just caught it funny, and it kind of rolled under me," Robertson said. "... I don't feel like it's something that's going to set me back a long time. We just need to let it get some of that soreness out of it right now."
He did not initially believe the injury was serious, but noticed a throbbing sensation about an hour later, prompting him to call head athletic trainer Steve Donohue.
"They were empty boxes; they weren't even heavy boxes," Girardi said. "I told him, 'Kick them down the stairs next time.'"
Girardi had said earlier in the day that he was not concerned about Robertson being unavailable for Opening Day, but the manager sounded less optimistic after the Yankees' 6-1 Grapefruit League loss to the Blue Jays on Thursday.
However, Girardi said it was much too early to begin preparing for a bullpen without Robertson. If Robertson were to miss a substantial amount of time, it is likely that right-hander Rafael Soriano would slide back into the eighth-inning role.
"I'm not so sure he's going to miss enough time that we won't have him on Opening Day," Girardi said. "Until we get the test results, I'm not going to know."
Robertson, 26, had a breakout season in 2011, registering a 1.08 ERA in 70 games while striking out 100 batters in 66 2/3 innings. He was an American League All-Star, pitching a scoreless inning in the Midsummer Classic in Phoenix.
"He's a big guy [in the bullpen]," Yankees closer Mariano Rivera said. "Last year he did a tremendous job. We're expecting something good out of him this year also."
Foot injuries to Yankees pitchers have proven costly in the past. In June 2008, while running the bases in Houston, Chien-Ming Wang suffered a sprained Lisfranc ligament and a torn tendon in his right foot, ending his season. It was considered responsible for negatively impacting the right-hander's career after he had posted back-to-back 19-win seasons.
"We've got to make sure he's healthy before we send him back out there," Girardi said. "A lot of times foot problems can affect your back, so we've got to make sure he's healthy before we send him back out there."
Bryan Hoch is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.