Gerard Butler ROCKS OUT Manhattan Club ... with Help from Oasis [VIDEO]

It hasn't been 2 months since Gerard Butler left rehab for substance abuse -- but last week, he was already back at an NYC bar, where he blew away the crowd with a knockout performance of "Wonderwall" ... and we got video!!!

Butler stopped by The Darby supper club in Manhattan last Thursday -- the actor didn't drink a drop ... but he did take the stage and perform several songs, including one by The Doors and another by The Rolling Stones.