Red Sox manager Bobby Valentine said that Youkilis, on the disabled list since May 2 with a lower back strain, would make a rehab start as the designated hitter for Triple A Pawtucket Wednesday in Durham, N.C.

“We’re trying to get him at least three at-bats as a DH,’’ Valentine said before Tuesday afternoon’s 5-0 win over the Mariners at Fenway Park. “He looked really good taking ground balls today, moving around, a bounce in his step. So he’ll start tomorrow, and we’ll get him enough [work], whatever it’s going [to take] to have him say that he feels good.’’

Once Youkilis makes that determination, the Sox will have a decision on what to do with rookie Will Middlebrooks, considered the third baseman of the future.

Since his May 2 call-up from Pawtucket, Middlebrooks is hitting .300. He has 14 RBIs in his first 12 major league games, tied with Mo Vaughn (1991) for the most by a Red Sox in his first 12 career games since at least 1918.

For now, though, the plan is to have Middlebrooks hold down third base while Youkilis continues to rehab.

“He’ll go from DH to playing in the field, and seeing some pitches,’’ Valentine said of Youkilis.

Doubront scare

Starting pitcher Felix Doubront had a scary moment during warm-ups.

Doubront was playing catch in front of the visitors’ bullpen in right field when he was hit in the back of the head, behind his right ear, by a fungo ball hit to Adrian Gonzalez, who was practicing shagging fly balls in preparation for an interleague series in Philadelphia this weekend.

Doubront went down in a heap and exited the field under his own power while being escorted by the medical staff.

“Whenever anybody comes off the field, you hold your breath,’’ Valentine said. “But he tested fine, he knows where he is, he knows where he’s going. Thank goodness, I think it just hit his ear and he’s going to be just fine.’’

Doubront appeared in the clubhouse before the game and said, “I’m OK,’’ before he retreated to the training room. Later he said, “Tomorrow will be the test, so we’ll see what happens.’’

Striking success

The only other career start Josh Beckett made on his birthday was a win six years ago against the Orioles when he threw seven innings of two-hit, one-run ball, striking out six.

Beckett, who turned 32 Tuesday, didn’t allow a run in seven innings in improving to 3-4. He allowed four hits, walking two and striking out nine.

He passed Bruce Hurst (1,043) for sixth place on the club’s all-time strikeout list and now has 1,049.

Asked about the apparent decline in Beckett’s velocity, which has dipped from a maximum of 94.7 m.p.h. when he first arrived with the Sox in 2006 to 91.5 this season, Valentine said, “There’s been times when his fastball has been plenty [good] to control the head of the bat.’’

First things first

In addition to Gonzalez shagging flies, David Ortiz took grounders at first base. “I think David could play first base,’’ Valentine said. “And Adrian looks all right in the outfield, too. But I think David could play first, and so does Dustin [Pedroia], so does Adrian, and so does David, and that’s what’s more important.’’ . . . The Sox completed the trade for Marlon Byrd by sending lefthanded pitcher Hunter Cervenko, who recently was part of Low A Greenville’s first no-hitter in franchise history, to the Cubs . . . Andrew Bailey, who started the season on the disabled list after undergoing surgery to repair a torn ulnar ligament in his right thumb, went to Cleveland for an examination. Valentine did not have any results of the examination after the game . . . A day after having his 14-game hitting streak snapped, Pedroia reached on a single to third in his first at-bat. He has now safely reached base in 15 of his last 16 games, hitting .330 with 10 doubles, 2 homers, 13 RBIs, and 18 runs over that stretch.