ANAHEIM, Calif. (AP) - The Boston Red Sox placed David Aardsma on the 15-day disabled list Sunday because of a groin injury, and recalled rookie Justin Masterson from Triple-A Pawtucket to replace him in the bullpen.

The Red Sox optioned Masterson to Pawtucket on July 7 with the idea of converting him into a reliever. He was impressive in the rotation, going 4-3 with a 3.67 ERA in nine starts, but the Red Sox have been looking for consistency from their middle relief corps as a bridge to closer Jonathan Papelbon.

Aardsma is 3-2 with a 2.75 ERA in 38 relief appearances. He gave up a walk and a two-run single to the first two batters he faced Friday night after relieving Clay Buchholz in the fifth inning of Boston's 11-3 loss to the Los Angeles Angels before retiring the final batter he faced.

"I'll make sure I give it a lot of rest so I'll be able to come back healthy," he said before the Red Sox met the Angels in the finale of a three-game series. "I know it's one of those things where the last thing I want to do is make it a lot worse than what it is."

Aardsma said he was injured July 9 while pitching against Minnesota.

"I feel it right at the longest point in my stride, right where I kind of explode," he said. "It just kind of grabbed me a little bit."