He hurt his thumb during batting practice yesterday while throwing a ball back underhanded towards the infield.
If Joba can't pitch, the Yankees will use Alfredo Aceves.
"We don't think it will be an issue (Sunday), but we had to protect ourselves," [manager Joe] Girardi said. "If he can't go, Ace would be the guy."In five starts, Chamberlain is 1-1 with an ERA of 3.77.