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."