Oakland Athletics pitcher Brandon McCarthy was struck in the head by a ball hit by Erick Aybar on Wednesday. (Doug Duran / AP)

McCarthy, who was struck in the head by a line drive hit by Erick Aybar on Wednesday, underwent two hours of surgery to relieve pressure on his brain. Although he never lost consciousness and walked off the field, McCarthy suffered an epidural hemorrhage, brain contusion and skull fracture in the accident.

McCarthy, 29, was described by the team as alert and in stable condition at an undisclosed hospital.

Dr. Geoffrey Manley, the vice chairman of neurological surgery at the University of California San Francisco, told Sfgate.com that bone fragments from a skull fracture can cut the middle meningeal artery, causing bleeding that creates pressure on the brain.

“If you are not treated for this, you could die, but if you’re treated rapidly, you usually have a very, very good recovery,” Manley said. “That is why people need to be evaluated promptly. Most patients who have this kind of injury return to a normal life. I have treated other athletes with similar injuries who have returned to playing sports.”

Brandon McCarthy is treated after being struck in the head by a line drive. (Doug Duran / AP)

Manley guided treatment for Bryan Stow, the San Francisco Giants fan who was in a coma after a beating in the Dodger Stadium parking lot last year. Manley indicated that, although patients can be discharged four or five days after surgery for epidural hematomas, the fracture must heal.

“[McCarthy] might not be back the rest of the season,” Manley said, “but I don’t think this would preclude him from playing baseball.”

The A’s, who had an off-day Thursday and play tonight in Seattle as they battle for a wild card, will have to adjust to the loss of McCarthy, “a cool, calm presence” in a young rotation. McCarthy is 8-6 with a 3.24 ERA.

“Our first concern is Brandon’s health, and we are heartened to learn he has shown progress in his recovery after surgery,” A’s General Manager Billy Beane said in a statement. “We are glad to report he is stable, awake and alert. ... Brandon remains in everyone’s thoughts as we wish him a speedy recovery.”

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McCarthy injured by line drive

MLB: Standings | Wild cards