9:51 AM PT -- A spokesperson for Tampa Police tells us they're asking the public to help identify the attackers ... adding the victim is in critical, but stable condition. Anyone with info is asked to reach out to Tampa Police.

A Nardo Wick fan got much more than he bargained for after asking for a photo with the rapper, as Nardo's entourage unleashed a brutal attack ... leaving the fan hospitalized and the incident under police investigation.

Sources with direct knowledge tell TMZ the 20-year-old fan attended the NW show Sunday night at Club Skye in Tampa, and spotted Nardo leaving the venue around 1:15 AM.

That's where video, obtained by TMZ, picks up ... you can see the fan approach Nardo and his team for what we're told was to get a photo. Then, without warning, a member of Nardo's team sucker punches the young fan ... knocking his head against a concrete wall.

Unable to move, the fan seems stunned ... and that's when another member of NW's entourage punches the fan again, causing him to fall to the ground and hit his head on the pavement.

The attack is cowardly, and Nardo -- wearing the white shirt near the rear of the SUV -- appears to try to get his guys to relax ... but the effort proves useless.

As for the fan, we're told he was taken to a local hospital where he was allegedly treated for a severe concussion, and there's some spots where he's hemorrhaging. Our sources say it's currently unclear if there will be any long-term damage ... only time will tell.

We reached out to a rep for Nardo Wick ... so far, no word back.