NBA star Nick Young has been named a suspect in an alleged robbery that went down last week -- in which Swaggy's accused of striking a guy in his testicles and stealing his phone. 

Law enforcement sources tell us ... the LAPD received a call on Jan. 4 from a man who claims he was assaulted by Swaggy P when he tried to take his photo at a popular L.A. car wash.  

We're told the alleged victim does not appear to be a paparazzo -- just a basketball fan who recognized the former Lakers star and wanted a photo. 

The man acknowledged Young didn't want to be photographed but he snapped a pic anyway -- and that's when Young snapped ... confronting the fan, allegedly firing off a nut shot and swiping the phone.