Jon Lester (lat strain) goes on DL

BOSTON -- The Boston Red Sox placed left-handed pitcher Jon Lester on the disabled list with a lower lat muscle strain Wednesday and recalled right-handed reliever Scott Atchison from Triple-A Pawtucket.

Lester suffered the injury and was removed after four no-hit innings Tuesday night against the Toronto Blue Jays.

"Jon Lester had an MRI (Wednesday) which confirmed the diagnosis of a latissimus strain," Red Sox medical director Dr. Tom Gill said in a statement Wednesday night. "He will be treated with a period of rest from throwing, followed by progressive strengthening program. There is no specific timeframe for his return at this time. His status will be better defined following the All-Star break."

The Red Sox made the move even before announcing the results of the MRI. Lester had been given a physical exam earlier in the day, but the medical staff thought it best to have an MRI to verify the diagnosis.

"We're happy that's what it is," Francona said of Lester's prognosis. "It was confirmed and he'll rest."

Before the results of the MRI were announced, a team source had told ESPNBoston.com that, depending on the severity of the injury, Lester could be sidelined for two to four weeks.

Manager Terry Francona said that even before the injury, he was thinking of skipping Lester's next scheduled start Sunday before the All-Star break and then not starting him again until July 19 at Baltimore. Now that he's on the DL, Lester won't be eligible to start until July 22.

"This is kind of a no-brainer," Francona said of placing Lester on the DL. "I've already talked to him today and he didn't put up much of a fight, so I think it's the right thing to do.

"I don't want to say he's upbeat. He's aggravated because you never want to miss starts, but I think he's pretty optimistic. He was reaching back (Tuesday) night and firing it pretty good. He has had problems in the past where he's cramped when it gets hot. Sometimes when it happens, you're sore the next day, and we're really hoping that's what it is. Even if that's what it is, we need to take it easy on him and get better, and that's why we're doing the tests."

Lester had been one of the two consistent starters in the Red Sox rotation, going 10-4 with a 3.31 ERA so far this season and challenging the American League leaders in wins, winning percentage and strikeouts. Atchison, 35, is in his fourth stint with the Red Sox this season; he is 0-0 with a 4.73 ERA.

Also, pitcher Clay Buchholz was examined by a back specialist Wednesday morning in North Carolina and has returned to Boston.

"Clay Buchholz was seen in consultation by Dr. Craig Brigham today in Charlotte," Dr. Gill said in a statement. "Dr. Brigham agreed with the team's assessment that the back symptoms can be treated with rehabilitation. Clay will be advanced in his activities as his symptoms allow."

"Buck will move ahead as tolerated," Francona said.

Francona didn't disclose his plans for the rotation going forward, saying such talk is "premature," but his options are getting pretty limited. In addition to having Lester and Buchholz on the DL, the Red Sox have lost Daisuke Matsuzaka (Tommy John surgery) for the season and gotten inconsistent work from John Lackey, who has a 7.47 ERA and has spent time on the disabled list himself with elbow issues.

However, Francona seemed encouraged by Wednesday's medical updates on Lester and Buchholz.

"Yeah, it's really good [news]," he said on both pitchers. "We thought Lester had that, but you've got to check because he's so good and so important it certainly feels good when you hear that."

Alfredo Aceves would seem a likely candidate to fill in for Lester on Sunday. Beyond that, the Red Sox have veteran Kevin Millwood and prospects Felix Doubront and Kyle Weiland at Triple-A Pawtucket.

Red Sox first baseman Adrian Gonzalez was given his first day off this season due to a stiff neck, according to Francona. Gonzalez told Francona after Tuesday's game that he was feeling it a bit, and since Gonzalez is 1-for-6 in his career against Toronto starter Ricky Romero, Francona thought it best to give the slugging first baseman the night off.

"He called me this morning and when he didn't put up a fight, sometimes guys need a day off," Francona said. "So it'll be good for him."

Kevin Youkilis, who missed Tuesday's game due to headaches and tightness in his trapezius after he was hit by a pitch Monday, was back in the lineup and playing first base for the first time this season.

"We'll let him play first tonight, and I checked with him and I wouldn't do it if he wasn't comfortable, but maybe this takes a little wear and tear off him for one night, too," Francona said before the game against the Blue Jays.

Youkilis went 3-for-4 with a home run and two doubles in the Red Sox's 6-4 victory.

Joe McDonald covers the Red Sox for ESPNBoston.com. Information from The Associated Press was used in this report.