Even before those results were in, the club already decided to place him on the 15-day disabled list.
The club is hopeful Lester can return to the rotation on his first day of eligibility, which would be July 22 at Fenway against the Mariners.
"We were actually planning on possibly skipping him on Sunday anyway and having him come out of the chute last just because of the load he will carry," said manager Terry Francona. "This kind of makes this a no-brainer. I already talked to him today. He didn't put up much of a fight, so I think it's the right thing to do."
To replace Lester on the roster, the Red Sox recalled right-hander Scott Atchison from Triple-A Pawtucket.
Already, Boston is without starting pitcher Clay Buchholz, who is on the disabled list with back woes and underwent a second opinion on Wednesday, which confirmed the club's opinion -- that rest and rehab should cure the injury.
"Clay Buchholz was seen in consultation by Dr. Craig Brigham today in Charlotte. 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," Red Sox medical director Tom Gill said in a statement.
As for Lester?
"Jon Lester had an MRI today which confirmed the diagnosis of a latissimus strain. 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," said Gill.
Daisuke Matsuzaka is out for the rest of the season after undergoing Tommy John surgery. John Lackey recently returned from an elbow injury, but he has been spotty all season.
The club hasn't named a starter yet to fill in for Lester on Sunday. Swingman Alfredo Aceves seems like the logical choice. Boston also has veteran Kevin Millwood at Triple-A Pawtucket.
Tim Wakefield, who earned the win Wednesday, and Andrew Miller, who starts Thursday, are already in the rotation as replacements for Buchholz and Matsuzaka.
At least with Lester, the Red Sox remain hopeful the injury -- sustained during four innings of no-hit baseball on Tuesday -- is just a temporary nuisance.
"I don't want to say [Lester was] upbeat, because I think he was aggravated," said Francona. "He doesn't ever want to miss starts or anything like that. I think though, [we're] pretty optimistic. He was reaching back last night and firing pretty good. He's had problems in the past where he's cramped, not just his shoulder, but legs, things like that, when it gets hot. Sometimes when it happens you're sore the next day. We're really hoping that's that what it is. Even if that's what it is, we need to take it easy on him and let him get better. That's why we're doing the tests."
In 18 starts this season, Lester is 10-4 with a 3.31 ERA. Buchholz, who last pitched for the Sox on June 16, is 6-3 with a 3.48 ERA in 14 starts.