Struck on the right side of his head by a hard shot from the Angels' Erick Aybar on Wednesday, McCarthy suffered an epidural hemorrhage, brain contusion and skull fracture. He had two hours of surgery to relieve pressure on his brain late Wednesday night.
A's athletic trainer Nick Paparesta was asked Friday if McCarthy was still in a life-threatening situation.
"Absolutely he is. It's brain surgery," Paparesta said. "It's life-threatening. At any possible moment, something could go wrong. He could have a complication. Absolutely."
Speaking before the team's game against Seattle, Paparesta also said McCarthy got out of bed three times, sat up in a chair, ate solid foods, recognized visitors, and although he can talk, he isn't talking yet because of continued pain.
Paparesta said the doctors and medical team have told him that the biggest issue with this type of injury is the patient's reaction the first two or three days.
"We're coming up on day three (Saturday), so by the evening time, there's still the possibility of reoccurring or increase of swelling back again," Paparesta said. "If that happens, they can do different measures or techniques to keep that down. He's not out of the woods, but he's showing great progress."
The 29-year-old McCarthy was 8-6 with a 3.24 ERA this season, his sixth in the big leagues and second with the A's. He was 9-9 with a 3.32 ERA last season.
"We definitely have some heavy hearts here," A's manager Bob Melvin said. "I don't think there's a guy on this team who's not close to Mac. It was hard to watch that at the time. Then you think to yourself, he's getting up and walking off, that's a good sign. But with head injuries, you just never know. The good news is each and every day he's getting better."
Paparesta said McCarthy's baseball career is his least concern.
"First and foremost, we need to look at his life and make sure he stays alive. This is serious stuff and we want him to live," Paparesta said. "I think the next phase we would go through is to look at his simple gross motor mechanics and skill set so to get him through his normal active life.
"For me, the way I look at it and the way and I think the doctors and medical staff looks at it, is we're worried about keeping him alive right now and making sure everything functions the way they need to be. Then we'll worry about the motor skills and going through normal daily stuff. Then later on down the road, we'll worry about his fine motor skills. We're just going day to day and making sure he's making progress."
Aybar talked about McCarthy before the Angels' game Friday night against Detroit.
"It has been a tough last two days for me as I keep replaying in my head when the ball struck Brandon," Aybar said in a statement released by the team. "I am encouraged and thankful to know he has made significant progress, and I keep praying to God to help him recover quickly and fully. I also pray for him and his wife. Our game is a tight brotherhood, and right now we all wear the same uniform colors."
Angels general manager Jerry Dipoto also issued a statement.
"From the moment Brandon was struck by that line drive, he has remained in the thoughts and prayers of not only our organization but countless others throughout the baseball community," Dipoto said. "That support will continue for Brandon and his wife, Amanda, as he begins the recovery process from yesterday's surgery. I am certain the discipline, commitment and inner strength Brandon has had to become an elite athlete will serve him well in the days and weeks ahead."