From MLB.com:

Hampered by a bone spur and plantar fasciitis in his left foot, Giambi has been unable to push off and drive the ball while batting, which has landed the 36-year-old slugger in a 4-for-24 funk entering Tuesday's game.

With that in mind, Giambi has successfully lobbied for the Yankees' medical staff to allow him to receive a cortisone injection, tentatively to be delivered during the team's Thursday off-day in New York.