DUNEDIN, Fla. -- Joe Girardi had immediate concern when he saw David Robertson in the clubhouse early Thursday morning.

And nothing that happened the rest of the day allayed that worry.

A freakish slip on a bottom step in the Robertson home Wednesday night caused a restless night for Girardi and the Yankees' organization the next day, as the extent of the star setup man's injury wasn't immediately clear.

"We're not sure what's going on," Girardi said late Thursday afternoon after the Yankees' 6-1 exhibition loss to the Blue Jays. "I would love to know what's going on, but I don't."

Robertson, who emerged as one of the premier relievers in baseball last year as Mariano Rivera's primary setup man, suffered what Girardi earlier in the day called "a mid-foot sprain" while moving some boxes in his St. Petersburg residence Wednesday night.

He was taken for X-rays, which came back negative, and an MRI Thursday morning.

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The results of the MRI gave the Yankees, Girardi said after 's game, a "pause for concern," and Robertson was sent back to the hospital for a CT scan and weight-bearing MRI. The results of those tests will be sent to the team physician, Christopher Ahmad, who will consult with Dr. Justin Greisberg, a foot specialist.

Results from the additional tests could come Friday.

"There's concern," Girardi said. "I was concerned this morning when I saw him walking in the clubhouse."

Robertson was in good spirits when he met with a group of reporters after returning from the first round of tests, though he was on crutches and wore a protective boot, items he said were "precautionary."

"Hopefully it's just a sprain and I'll be back in a week or so," Robertson said, speaking before he was sent back for more tests. "I don't feel like it's something that's going to set me back for a long time."

Of the accident, Robertson said: "I just misjudged one step and just kind of caught it funny and it kind of rolled under me. I just had an empty box. It's not like I was carrying 70 pounds. I was just taking it to go put in the recycling."

In the morning, Girardi said that if Robertson is sidelined more than two weeks, it might put his ability to be back by the start of the regular season in jeopardy.

The biggest fear, however, is that the reliever, who went 4-0 with a 1.08 ERA in 70 appearances last season, will have to miss a much bigger chunk of time.

If Robertson is out for an extended period, Girardi said, Rafael Soriano will take over as the eighth-inning man.

Many believe the Yankees have the best bullpen in baseball, and although the unit does appear deep, losing Robertson for an extended period would be a blow.

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Robertson, who turns 27 in April, held opponents scoreless in 63 of 70 appearances. The All-Star led all American League relievers in ERA and ranked second in the majors behind the Braves' Eric O'Flaherty (0.98), allowing the Yankees to weather injuries to Soriano, who started the year as Rivera's setup man, and Joba Chamberlain.

If Rivera does retire after this season -- something the 42-year-old closer has hinted he'll do -- Robertson, with another good year in 2012, probably would get first crack at replacing him.

The injury, the circumstances of which Robertson called "embarrassing," brought to mind Chien-Ming Wang, another Yankees pitcher who had foot issues.

Wang suffered a torn Lisfranc ligament in his right foot while running the bases in an interleague game in June 2008 in Houston and was never the same pitcher afterward.

"Obviously, you have to be concerned about that," Girardi said in the morning, referring to the possibility of a Lisfranc tear. "There was no swelling, which was a positive sign to me."

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Girardi paused and asked what turned out to be the question of the day:

"But who knows?"