Boston Red Sox starter Justin Masterson had trouble keeping his sinker down last night against Arizona.

The Providence Journal / Gretchen Ertl

BOSTON - Justin Masterson has been in the big leagues for only seven starts now.

No one doubts that the rookie right-hander has the stuff and the maturity to become a winning pitcher in the majors.

The key for him, though, is to make sure he has a good sinker to go along with a sharp slider. If Masterson is getting a lot of ground-ball outs, he’s on his game. and If the ball is up, his success is going to be more problematic.

Last week in Philadelphia, Masterson had difficulty with his command in stretches, leaving the ball up. Backed by a big early lead, however, Masterson was able to struggle his way through five innings, leaving with a 7-2 lead in an eventual 7-4 Boston victory.

Last night, Masterson again had trouble keeping his sinker down. Couple that with the inconsistency he had with his slider, and it’s little wonder he seemed to be pitching out of the stretch an awful lot.

Of the 18 outs recorded by Masterson, eight came on ground balls. Masterson struck out four, so the outfielders had to haul in six fly balls/line drives.

That isn’t a bad ratio, necessarily, but another indication he was up with his pitches.

Not one of the Diamondbacks’ seven hits off Masterson was a ground-ball base hit. The base hits didn’t touch the ground until reaching the outfield or, in the case of Chad Tracy’s three-run homer in the third, until it clanged off the back of the Arizona bullpen fence.

This is not to say that Masterson wasn’t able to find his sinker every now and then. In the fifth, for instance, facing a first-and-second, one-out jam, Masterson threw a sinker to Chris Young, a good one, that Young banged on the ground for a double play.

And in the sixth, his final pitch, a sinker, was grounded into an inning-ending force-out.

Twice during his outing pitching coach John Farrell sauntered out of the Boston dugout to offer some words of advice. No doubt they’ll be working before Masterson’s next start to make sure his mechanics allow him to get the consistent “sink” on the ball he needs.

skrasner@projo.com