Bay likes what he's seen. And, to be sure, the Red Sox are also pretty pleased about what they've witnessed thus far from their new left fielder.
Lester (12-4) allowed one run, four hits and a walk in seven innings to improve to 5-0 lifetime against the Orioles. The left-hander is 9-1 since May 25.
"I don't think my mind-set has changed any since April," Lester said. "More consistent, pound the zone. If you can show guys you're ahead in the zone, they're going to swing at pitches early in the count that aren't necessarily good pitches to hit. I was able to do that tonight."
"I'll be honest, I didn't know a lot about him before I got here," Bay said of Lester. "But it's pretty obvious to me after being here that he's one of the better pitchers in this league."
Bay, meanwhile, is quickly becoming one of the AL's most dangerous hitters. In 16 games with the Red Sox, Bay is batting .348 with three homers and 16 RBIs.
"That's part of my job, driving in runs, and you've got to have guys on base to do that," he said. "It's a testament to those guys. [I'm] just trying to be a complement to the puzzle."
Jason Varitek also homered for the Red Sox, who have won eight of 12 and lead the AL wild-card race.
Manny Delcarmen issued two two-out walks in eighth before Jonathan Papelbon gave up a double to Aubrey Huff that cut Boston's lead to 4-3. After David Ortiz hit an RBI double in the ninth and scored on a grounder by Bay, Papelbon finished for his 33rd save in 37 tries.
Guthrie (10-9) gave up two runs and five hits in seven innings, but was victimized by a familiar lack of support: Baltimore has totaled 14 runs in his nine losses. The right-hander had won each of his previous four starts, permitting only one run in each.
In this one, however, Guthrie matched his season high with five walks and found it difficult to keep pace with Lester.
"Probably the biggest struggle I've had all season with my stuff," Guthrie said. "My fastball command was not there; I walked a lot of guys. It might have been the toughest outing of the season for me as far as laboring. I had to work hard to get through five and get a couple more."
Huff hit his 27th home run, drove in three runs and finished with two of Baltimore's five hits.
"We've been coming back all year long and so were expecting one of those games," Huff said. "You're not going to win them all, but obviously that's one of the better teams in the American League East and the American League for that matter. They've got a great bullpen and Lester gave them a great start tonight."
Bay homered leading off the second, and Varitek added a long drive over the scoreboard in right later in the inning to interrupt a lengthy slump. Boston's captain was batting .164 with only two home runs in his previous 63 games.
Huff connected in the fourth to get Baltimore to 2-1. His second homer in two games left him seven short of matching his career high, set in 2003 with Tampa Bay.
Except for that hit, the Orioles didn't get a runner into scoring position until the seventh, when Kevin Millar hit a one-out double. Luke Scott then struck out on a 3-2 pitch and Jay Payton grounded out.
Boston went up 4-1 against Rocky Cherry in the eighth. After Millar booted a grounder to first for an error, Bay hit his 25th homer of the season, 22 with the Pirates.
Attendance was 40,429, leaving the Orioles 2,861 fans shy of reaching the 50 million mark at Camden Yards. ... Boston is 7-15 against the AL East on the road, including 4-3 against Baltimore. ... It was Bay's 12th career two-homer game, the first as an American Leaguer.