Brewers center fielder Mike Cameron rocketed a line drive that hit rookie reliever Joe Martinez on the right side of his head, near his temple.
Martinez, who was knocked down, initially stood before crumpling to the ground, his head bleeding, as team trainers raced to the mound. After a few minutes, he walked off the field, with trainers by his side, as fans gave him a standing ovation.
"We're going to take him out to the hospital, evaluate him and get a CT scan," manager Bruce Bochy said. "He's conscious. He's aware of his surroundings. It caught him off the top of the right forehead.
"It's always a scary moment. It makes you sick when that happens."
Bochy said that Martinez's head was "bleeding quite a bit" and that his right eye is swollen.
Late Thursday night at St. Francis Memorial Hospital, Giants executive Bobby Evans told the San Francisco Chronicle that Martinez "has a concussion but otherwise seems to be OK."
"Evans did not want to get too specific, medically, but said that Martinez did not appear to have any broken bones, and tests did not reveal any serious injuries," the Chronicle wrote. "But they are keeping (him) in the hospital overnight for observation. You can't be too careful with head injuries."
The Chronicle added, "Evans said Martinez was up and talking and did not even seem to have a bad headache. Martinez spoke by phone with his parents in New Jersey, who unfortunately watched on TV as their son got hit."
Cameron, for his part, looked away in horror after the hit. He pulled into second and hung his head, unable to watch. Cameron, after all, had suffered multiple facial fractures in August 2005 while a member of the New York Mets, when he collided with Carlos Beltran while the two outfielders dived for a fly ball in San Diego's PETCO Park.
"I hope the young man is all right," Cameron said. "I hope he's safe. I hope he gets back on the field again. It was one of those moments where I just had this helpless feeling. I've been in a similar situation, and I know all kinds of things go through your mind. I'm praying for him."
Players from both teams, those in the game and on the bench, were shaken by the violent scene.
Giants left fielder Fred Lewis went over to Cameron while Martinez was being treated on the mound. Lewis could relate. In 2004, during a Minor League game, he hit a line drive that struck current A's reliever Brad Ziegler in the head. Ziegler spent six days in intensive care and nearly died.
"I couldn't get out of the batter's box," Lewis said. "It's one of those things you never want to see."
Lewis said that Cameron was shaking. So was Giants catcher Bengie Molina, some 20 minutes after the game.
"Before baseball players, we're human beings," Molina said. "We have families, we have brothers, sisters, dads, moms, sons. You don't want to see that on the baseball field."
Molina said that Martinez didn't have a chance to get his glove up and protect himself. The ball was hit too hard.
"It's unbelievably scary, because it can happen to any pitcher," Brewers starter Manny Parra said. "Balls go flying by your head all the time in every game. It's extremely unfortunate. That's something you don't want to watch."
Giants starter Matt Cain was in the clubhouse when Martinez was hit but saw him after he came into the trainers' room.
"He was conscious and everything," Cain said. "It looked like he was talking. Not really talking, but definitely coherent, and could tell kind of what was going on."
Eric Gilmore is a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.