From the official site:

Jason Giambi received some better news regarding his aching right foot on Wednesday. The results of his midafternoon MRI came back negative, revealing just a bone bruise.

Giambi, 36, was hit on the top of the foot by a pitch on Monday, against the Orioles. Though he didn't leave the game, he reported to Yankee Stadium on Tuesday unable to start as expected and had his foot heavily wrapped before leaving the park on Wednesday. ...

X-rays taken on Monday revealed a contusion, but the swelling two days later raised the question of whether it may be something more. Results of the MRI performed at the Columbia Presbyterian Medical Center officially leave his status as day-to-day.