The Red Sox had won nine of their last 11 games to climb back to .500 before the O's notched a 4-1 win on Tuesday. With the victory, Baltimore holds on to an unfamiliar two-game lead in the division, while Boston drops to an equally unusual last place.
Boston will send Daniel Bard to the mound to try to earn a victory in the rubber match, while Baltimore will counter with Jake Arrieta.
Bard struggled in his most recent outing, allowing five runs in five innings to take a loss against Philadelphia.
Bard said he was inconsistent with his mechanics, the reason he allowed five walks in the defeat and threw only 49 of his 94 pitches for strikes.
"When you get out of your delivery a little bit, the best thing to do is step off, take a breath and hit the reset button," Bard said. "I never did that. I tried to power through it."
A lack of control has been Bard's biggest problem as he attempts to make the transition from reliever to starter. His 5.27 walks per nine innings is among the most in the Majors.
Arrieta, on the other hand, has thrived by cutting his walks per nine innings in half -- from 4.45 in 2011 to 2.11 this season.
Arrieta credited the turnaround to offseason surgery to remove a bone spur in his elbow, increasing his flexibility and range of motion. In the past, the discomfort caused by the bone spur led to him losing command of his pitches over the course of a game.
"It helps me prolong my command throughout the entire outing," Arrieta said. "In the first few innings my command was usually pretty darn good, and then around the third, fourth, fifth inning, I just lost feel for breaking balls and command of fastballs."
Orioles: Hardy getting hot
After hitting just .184 with three home runs and six RBIs in April, J.J. Hardy has started to turn it on in May, hitting .305 with six homers and 15 RBIs.
With a walk in the first inning Tuesday night, Hardy has reached base safely in 25 of his past 28 games.
Adam Jones extended his hit streak to 14 games with a third-inning single on Tuesday, tying Nolan Reimold for the longest hitting streak by a member of the Orioles this season.
Red Sox: Youkilis thrives vs. Orioles
Third baseman Kevin Youkilis crushed a fourth-inning home run on Tuesday in his first game back from a low back strain.
Youkilis has been very successful against the Orioles in his career, hitting .320/.425/.573 against them with 21 home runs and 83 RBIs in 102 games.
The Red Sox lead the Majors with 174 extra-base hits, including a Major League-best 116 doubles.
With his 0-for-4 performance Tuesday, Dustin Pedroia snapped a 20-game hitting streak against the Orioles.