BOSTON -- The Red Sox have placed third baseman Kevin Youkilis on the 15-day disabled list because of what manager Bobby Valentine called a lower-back strain, and promoted Will Middlebrooks from Pawtucket to take his place.

Valetine also announced that Josh Beckett will miss his next scheduled start, Saturday against Baltimore, due to stiffness in his right lat muscle and that PawSox pitcher Aaron Cook will take the mound instead. Cook, whose contract called for him to have the chance to leave the organization as a free agent if Boston didn't promote him from Pawtucket by May 1, has remained with the team and will be added to the major league roster on Saturday, according to Valentine. A corresponding move will have to be made to make room for him in Boston.

Aaron Cook will start Saturday against Baltimore in place of Josh Beckett, who has stiffness in his right lat muscle, according to manager Bobby Valentine. AP Photo/Charles Krupa

Valentine said he thinks Beckett will be able to start again when his turn in the rotation comes up the next time through.

He also mentioned another possibility for the Sox rotation.

"I'm not opposed to a having more than five starters during long stretches depending on how the pitchers are pitching," said Valentine. "There's no need to project anything further (regarding Cook's status) than Saturday."

Middlebrooks, a hot prospect who was hitting .333 with a team-leading nine homers and 27 RBIs for the PawSox, was in Wednesday night's lineup against Oakland, batting eighth and playing third base.

"He's been hot in Triple-A and played very well in spring training," said Valentine. "I told him to have fun, enjoy it, get into the moment. He's good at that from what I saw in spring training."

Middlebrooks, 23, a fifth-round draft pick in 2007, was excited to get the call.

"It's unbelievable, something you work toward your whole life," he said. "I worked to be here. I'm ready to go. I'm feeling really comfortable at the plate and the same way on defense."

Youkilis, meanwhile, had kind words for his temporary replacement.

"It's a great opportunity for him to come up and play at this level. It's an exciting time for him. He'll remember this night for the rest of his life. I hope he does well. It's another level, but he can play at this level. He's a great guy and a great player. I'm hoping he can help the team win," said Youkilis.

Youkilis played last Saturday in Chicago, but was unable to get loose during batting practice on Sunday, so he had missed three straight games before being disabled. He said Wednesday that the source of his troubles has been identified, and that he felt much better Wednesday than he had on Tuesday,

Youkilis, 33, has been bothered by various nagging injuries this year, beginning in spring training. He had played in 18 of the Sox' 23 games this season, and was batting only .219 with two homers and nine RBI. Injuries limited him to only 102 games in 2010 and to 120 last year.

"It definitely stinks. It's frustrating. I don't want to be injured. I want to play. There's nothing I can do but get healthy and help the team win," said the eight-year veteran, who is in the final year of his contract.

In another Red Sox move, right-handed pitcher Clayton Mortensen was promoted from Pawtucket to give Valentine another arm in the bullpen. He replaced first baseman/outfielder Lars Anderson, who was sent back to Pawtucket after Tuesday night's game.

Mortensen, who was obtained from Colorado in exchange for Marco Scutaro over the winter, will provide depth out of the bullpen for the time being. He was 2-2 with a 0.90 earned-run average in six games, totaling 10 innings.