Carlos Beltran, listen up … Bobby Valentine's got some Mets advice, telling TMZ Sports if the new NY manager wants to win now — FIX THE BULLPEN ASAP!!!

The Mets were a disaster at the end of games last season — closer Edwin Diaz blew SEVEN saves — and that ultimately led to Mickey Callaway's firing last month.

When we got Valentine out at Mariano Rivera's charity event in NYC … the ex-Mets manager told us if Beltran wants to avoid the same fate, clean that part of NY's roster up NOW!!!