Cook was making his debut for the Red Sox when the incident occurred. He stayed in the game, but he was shelled (2 2/3 innings, eight hits, six earned runs) and removed shortly thereafter.
To replace Cook on the roster, Boston summoned lefty Andrew Miller from Triple-A Pawtucket, where he had been in the final days of a rehab assignment.
"We need a little help in the bullpen, obviously," said manager Bobby Valentine. "So Aaron understands that. He understands baseball. [Plus], he has 11 stitches, and I can't try to have him pitch. He's had shoulder problems in the past. If the stitches come out, there could be infections. It's the only thing we can do at this time, I think. He'd like to pitch. But it's not going to happen."
Cook didn't have a defined role on the pitching staff. He was filling in for Josh Beckett for just one start while the ace righty battles a lat injury.
Miller, who pitched for the Red Sox in 2011 as both a starter and reliever, opened the season on the DL with a strained left hamstring.
He was used exclusively as a reliever at Pawtucket, where he posted a 5.73 ERA in 10 outings.
"I've been throwing the ball well," said Miller. "It took a while to get back up here. I feel like I used up almost every day available [of the 30-day rehab], but I'm up to help contribute."
Miller becomes the third lefty in Boston's bullpen, joining Franklin Morales and Rich Hill.
"He'll be our first left-hander out of the bullpen, and I'd like it to begin with getting left-handed hitters out," said Valentine.