The Red Sox have had some great years. This isn't one of them. Boston has a worse record than Baltimore.
So I'm not buying the Red Sox being this large of a favorite against the Orioles in a pitching matchup of Alex Cobb versus lefty Martin Perez.
Both are lower-tier rotation type starters. Cobb was solid in his last outing giving up two runs on four hits in six innings this past Friday against the Rays. He's been solid, too, in two starts versus Boston this season giving up a combined three runs on 10 hits in 10 1/3 innings. 
Cobb's 4.76 season ERA is lower than Perez's lifetime ERA against the Orioles. 
Note, too, that the Orioles are a .500 team when going against a southpaw starter.