However, the move was temporary, and made primarily to increase roster flexibility amid a seven-game stretch in which the Red Sox will face a tough batch of left-handed hitters from the White Sox and Yankees.
Lefty specialist Javy Lopez was summoned immediately from Triple-A Pawtucket to take Lester's spot on the roster. Lester will miss just one turn in the Boston rotation and make his return on Sept. 2 against the Orioles, at which time rosters will be expanded to 40.
The White Sox feature quality left-handed bats such as Jim Thome and Darin Erstad while the Yankees have Bobby Abreu, Hideki Matsui, Jason Giambi and Johnny Damon.
The only reason Lester went to Portland instead of Pawtucket is so he could keep his regular rotation spot and make his lone Minor League start on Sunday.
"We wanted to figure out a way to get Javy Lopez back here because he's a good part of our bullpen," said Red Sox manager Terry Francona. "And having [Hideki] Okajima as the only lefty -- he gets outs wherever you put him in the game. It will be helpful to have the other lefty. This way, Lester can pitch on his normal turn, come right back and sort of let us have a little bit larger roster than 25 even though it's 25. Tavarez can pitch Sunday and hopefully having two lefties will help us."
It was Tavarez who was bumped from the rotation when Lester initially re-joined the team. But Tavarez has remained stretched out, making a pair of spot starts in August, the most recent of which occurred Sunday against the Angels. In that one, Tavarez surrendered two hits and two runs over six innings in a tough-luck loss.
After pitching well in that first start on July 23 in Cleveland, Lester (2-0, 5.67 ERA in six starts) has been inconsistent thereafter.
Francona emphasized that the club is not discouraged by the way Lester has pitched.
"[Pitching coach] John Farrell and I talked to him at the hotel this morning and kind of explained what we were doing just to make sure he understood that he is coming right back and starting and that if he goes and has a good start, that can't do anything but help," Francona said. "I don't think we said we were disappointed in his performance. I think, like a lot of young pitchers, there's some inconsistencies and, again, he's a guy who had his development halted, which doesn't make it easier."
Lopez has spent the majority of the season with the Red Sox, going 2-1 with a 2.87 ERA in 44 appearances.
Pedroia in, then out: Dustin Pedroia was originally in Thursday's lineup, but later scratched by Francona because of recurring stiffness in his left elbow. Pedroia was plunked on the left elbow by an Edwin Jackson pitch in Wednesday's game.
With Pedroia out of the mix, Francona tweaked the lineup and moved Coco Crisp to the leadoff spot. Julio Lugo moved from ninth to eight. Alex Cora, making a rare start against a lefty, filled in for Pedroia at second and batted ninth.
Reserve Eric Hinske might be unavailable the next couple of days after injuring his right calf while running the bases on Wednesday.
Clayton on board: In a move that was basically an insurance measure in the event of an injury, veteran infielder Royce Clayton was signed to a Minor League deal by the Red Sox and will join Pawtucket on Thursday.
There's a good chance Clayton will join the Red Sox when rosters are expanded on Sept. 1. Francona said Clayton would not cut into invaluable reserve Cora's playing time.
"My understanding is that we always call up a middle infielder," Francona said. "It's more insurance. We fight so hard to get Alex playing time that I don't want to take away from that and I've already talked to Alex about that. Again, if it's appropriate at some point, I would certainly talk to Royce. But we might be getting a little ahead of ourselves."
Mirabelli progressing: Backup catcher Doug Mirabelli is recovering nicely from the right calf strain that put him on the disabled list on Aug. 17.
"He had a good day [Wednesday]," Francona said. "Doing the elliptical, no swelling, he's actually doing pretty well. I think he was going to go in today and throw and swing the bat a little bit. He's doing pretty well."
Mirabelli is eligible to return from the disabled list on Sept. 2.
A worthy cause: For those Red Sox fans who are either based in Chicago or in the Windy City for this weekend's four-game series, there is a charitable event scheduled that is worth considering for Saturday night.
Gordon Edes, the veteran beat reporter for The Boston Globe, is hosting an event called "Edes and Friends" at the Rock Bottom Restaurant and Brewery Restauarnt on 1 W. Grand Avenue in Chicago. It is accessible via the red line for those fans coming directly from the Red Sox-White Sox game.
Francona is the special guest at the event, which benefits the cancer-fighting Jimmy Fund. There will be several coveted auction items -- both live and silent -- including tickets for the Red Sox, Patriots and Celtics. The event takes place between 8-11 p.m. CT.
For information on how to get tickets, go to www.jimmyfund.com/eve/event/gordon-edes/default.html.
Coming up: Curt Schilling (7-5, 4.25 ERA) will face Jon Garland (8-9, 4.89 ERA) in Friday night's second game of this four-game series. First pitch is scheduled for 8:11 p.m. ET.
Ian Browne is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.