Red Sox Star Clay Buchholz -- The Biggest Rock Star On the Team Is ...

Clay Buchholz and the Boston Red Sox have landed in NYC for their series with the Yankees -- but not without their trusty instruments ... and Clay tells TMZ Sports there's one guy on his team who's got serious rock star skills.

Clay is a huge fan of the guitar -- telling us he plays before and after he takes the mound because it helps clear his mind.

But he's not the only picker on the team -- Clay tells us there's a whole bunch of guys who jam together ... but there's one guy in the rotation who's not only killer on the strings, but can SING too.