Josh Beckett revealed on Friday that he went into his start against the White Sox last Sunday with pain in the latissimus muscle behind his right shoulder. The injury worsened after he threw 126 pitches on a cold day.

“I wanted to pitch and I probably should have been a little smarter before my start and maybe brought something up,’’ Beckett said.

Said manager Bobby Valentine, “In retrospect, it would have been nice to know that.

“We have a system set up to have information flow. It seemed like there was a little snag there. It’s not up to him to come into the manager’s office. You talk to the pitching coach all the time and trainers all the time. That information usually flows up.’’

Beckett said the muscle was giving him problems for about a week but he went ahead and pitched, giving up three runs over 6 2/3 innings in a game the Red Sox lost, 4-1.

“You can always on your start day make yourself believe you’re better than you are,’’ Beckett said. “I wasn’t very smart about it.’’

Now Beckett will miss his start against the Orioles on Saturday. But he expects to return to the rotation next week.

“I’m pretty optimistic that this next start is going to be fine,’’ said Beckett, who played catch from 90 feet before Friday night’s game. “Felt a lot better today than I did the day before my last start.’’

Beckett will play catch again on Saturday, then start his usual pre-start routine. He agreed that missing a start made sense.

“They didn’t really give me a choice in it,’’ Beckett said. “Bobby came in the training room and said, ‘Hey, you’re going to miss a start.’ It wasn’t totally shocking when he came in there and said it. It was something that was discussed before that.’’

Change of plans

Daisuke Matsuzaka did not make his scheduled start for Triple A Pawtucket Friday because of pain on the left side of his neck.

He is now tentatively scheduled to start for the PawSox at McCoy Stadium Monday against Rochester. Matsuzaka threw a 45-pitch bullpen session at Fenway Park on Friday.

“Nothing to do with his throwing, he felt great throwing,’’ Valentine said. “Rather than having him pitch with any kind of ailment, we got it better.’’

Matsuzaka, who is recovering from Tommy John elbow surgery, has made two minor league rehab starts and will make at least two more before being cleared to return to the majors.

If all goes well, Matsuzaka could be ready around the middle of the month. His rehab assignment could go 30 days, however. That period ends May 23.

Cook ready