Jon Lester will be in the minors until his recall by the Red Sox to start on Sept. 2.

AP / Mike Carlson

CHICAGO — In a bit of roster sleight-of-hand, the Red Sox made an unexpected internal move yesterday to prep them for the next two series, highlighted by the upcoming three-game showdown with the Yankees.

The Sox optioned starting pitcher Jon Lester to Double-A Portland and promoted lefty reliever Javier Lopez before last night’s game against Chicago was postponed because of rain and rescheduled as part of a day-night doubleheader today.

Lester’s stay in the minor leagues will be brief. Set to start the series finale here Sunday, Lester will instead pitch for the Sea Dogs Monday, then return to the big leagues in time to start Sept. 2 after staying in the minors for the mandatory 10-day minimum.

In turn, Julian Tavarez will take Lester’s spot here Sunday, making his second spot start in the last week. Tavarez started last Sunday against the Los Angeles Angels of Anaheim, and while he was saddled with the loss, he pitched well, allowing just two runs on two hits in six innings. “We tried to go through a lot of scenarios,” said manager Terry Francona. “We wanted to figure out a way to get Javier Lopez back here and part of our bullpen. It’s helpful to have that other lefty beyond (Hideki) Okajima.”

The timing was critical. The White Sox have a number of lefty hitters in their everyday lineup, including outfielders Darin Erstad, Jerry Owens and Scott Podsednik, catcher A.J. Pierzinski and DH Jim Thome.

But the real value in having Lopez comes next week when the Sox travel to New York. The Yankees’ lineup features lefties Johnny Damon, Hideki Matsui, Robinson Cano, Bobby Abreu and Jason Giambi, along with switch-hitters Jorge Posada and Melky Cabrera.

Francona didn’t relish the prospect of a three-game series with just one left-hander at his disposal out of the bullpen.

Lopez, who spent most of the first four months with the major-league club, was optioned back to Pawtucket earlier this month when the team needed a roster spot for the activation of Curt Schilling.

Lopez was frustrated by the decision, but understood that because he had options remaining, he was merely the victim of a numbers game.

Francona promised him he would return, though at the time it was thought that he might have to wait until rosters expand on Sept. 1.

In 44 games with Boston this season, Lopez was 2-1 with a 2.87 ERA. While with Pawtucket, Lopez was 2-1 with a 3.17 ERA and had made seven consecutive scoreless appearances prior to his promotion.

The PawSox were in Buffalo yesterday and because of weather issues throughout the Midwest, Lopez’s arrival was delayed here. He eventually arrived at 8:45p.m. Central time.

In announcing the move, the Sox were careful not to have it interpreted as a loss of confidence in Lester, whose performance has been spotty in six starts (5.67 ERA).

“We’re not disappointed in him,” said Francona. “But like a lot of young pitchers, there’s been some inconsistency. (But) this is just one start.”

Lester’s up-and-down nature was on display in his last two starts. After shutting down the Devil Rays on two hits and a run in seven innings on the team’s last homestand, he gave up five runs in 5 1/3innings earlier this week against the same Tampa lineup.

The series of moves leaves the Sox without an obvious starter for Sept. 1 against Baltimore. Tavarez could make that start, too, operating with five days’ rest.

It’s more likely, however, that the team will promote Clay Buchholz, who beat the Angels last week in his major-league debut, to make his second spot start.

Buchholz was to receive a promotion back to Boston when rosters expand, anyway, and the open date gives the team a second opportunity to evaluate him in the starter’s role.

smcadam@projo.com