Wednesday afternoon, Brandon McCarthy was struck on the head by an Erick Aybar line drive, and the initial prognosis was positive after he walked off the field under his own power. But after a CT scan last night, McCarthy underwent brain surgery to clear up a hemorrhage and repair a skull fracture. From the A's official Twitter account:

RHP Brandon McCarthy, who was struck on the right side of the head by a batted ball yesterday, underwent surgery last night.

The surgery included the evacuation of an epidural hemorrhage and stabilization of the skull fracture.

The surgery lasted nearly two hours. McCarthy underwent another CT scan on Thursday, which showed the condition had improved and the hemorrhage had subsided. According to the A's, he is alert and awake, and he is resting in the critical-care unit of his hospital.

Dan Straily, who was scheduled to start for the Sacramento Rivercats in the first game of the Pacific Coast League playoffs, is likely to replace McCarthy on the active roster. But, obviously, that is a minor concern at this point, as McCarthy's injury seems to have been far more serious than most observers thought.