From the official site:

Phil Hughes rolled his left ankle while performing conditioning exercises at the Legends Field training complex on Friday, and will have an MRI performed to determine the severity of the injury.

"He was doing his agility drill and stuck a spike; [the] spike caught," manager Joe Torre said. "They don't think it's anything, just rolled it a little bit. It may set him back a couple of days." ...

General manager Brian Cashman said that Hughes will be sent for an MRI on either Friday or Saturday. Torre said that the injury is not expected to significantly alter Hughes' projected return date.