BOSTON - There is very little margin for error for a guy like Doug Davis whose fastball lights up the radar gun at only 85-86 mph.

He has to be precise with his location and avoid falling behind in the count or finding himself in situations where he has to throw a strike.

Such was the case in the first inning. The count ran full on Dustin Pedroia, the second batter to face Davis. The Diamondbacks’ left-hander didn’t want to walk Pedroia, so he “challenged” him with his fastball, an 85-mph pitch down the middle.

Thank you very much, said Pedroia, crushing the mediocre fastball off one of the canvas signs atop the Green Monster in left for a homer and a quick 1-0 lead.

Davis has a funky, herky-jerky delivery, but his motion didn’t initially fool the Sox, who banged out three hits in the first and were robbed of another.

After the first inning, though, Davis went to more off-speed stuff, mixing in curveballs (69-71 mph), changeups (76-78) and cutters (81-83) to great effect.

Davis’ fastball never pushed past 87 mph, but the mix of pitches befuddled the Sox’ lineup, watered down because of injuries and Sean Casey’s suspension. Boston managed only five hits and the one run off him through seven innings.

Davis was lifted after giving up a single to each of the first two batters he faced in the eighth.

skrasner@projo.com