Damian Lillard's exploding rap career just got it's first legendary co-sign ... from Common ... who says Dame's skills on the mic are so FIRE ... he'd be down to work with him.
Common was leaving Equinox when we asked him about Lillard's rhyming ... wanting to know if the NBA star actually has the goods when it comes to hip hop.
A bunch of labels think so ... we already told you Dame's being courted by HUGE record companies ... and he's currently mulling his options.
Common -- himself one of the biggest names in rap -- tells us he's heard Dame spit and and thinks he's absolutely ILL .