Lester won for the fifth time in his previous six starts last Monday against Tampa Bay, as he held the Rays to a pair of runs and four hits in six innings to run his record to 16-8, while lowering his earned run average to 3.26. He also struck out 10 batters for the third straight start.
The 26-year-old left-hander, who is a native of Tacoma, Washington, lost to the Mariners the last time he faced them, and is 1-2 with a 4.58 ERA in six starts.
Boston salvaged the finale of its three-game series with the Oakland Athletics on Sunday, as J.D. Drew finished 3-for-3 and knocked in two runs in the Red Sox' 5-3 win. Ryan Kalish added a pinch-hit, two-run single for Boston, which won for just the third time in its last nine tries.
Boston is now eight games back in the American League East and 7 1/2 behind in the wild card race.
"Obviously, we know we're in a deficit," said Kalish. "We all know that. But to get a boost, to get on a plane and have the rest of the night to come back tomorrow and regroup for a big series. ... Obviously, every game means everything right now, so it was really good to get this win."
Josh Beckett (5-4) earned the win by allowing three runs on five hits with seven strikeouts over six innings.
"He made some big pitches with men on base to wiggle out of jams," said Boston manager Terry Francona on Beckett. "He gave us a chance to rally and give us a good comeback."
Seattle, meanwhile, continues to play out its string and enters tonight's tilt riding a four-game losing streak after getting swept by the LA Angels of Anaheim over the weekend.
The Mariners have lost five of six games overall and will pin their hopes tonight on righty Doug Fister, who is 5-11 with a 3.84 ERA. Fister defeated the A's his last time out, holding them to a pair of runs and five hits in five innings.
Fister lost to the Red Sox earlier in the year and is 0-1 in two starts against them with a 3.18 ERA.
Boston is 4-3 against Seattle this season.