Sources tell TMZ Sports a head injury is forcing Gib to back out of the bout at Crypto.com Arena ... after doctors refused to clear the YouTuber to step into the ring.

We're told McBroom is scrambling to find a new opponent ... but it's not looking good.

If he's unable to get a fighter, sources say the event is at risk of being scrapped ... which would be bad news for people wanting to see two NFL running backs try to knock each other out.

We broke the story -- Le'Veon Bell and Adrian Peterson are slated to face off in the ring next weekend ... as well as rapper Blueface and ex-NBAer Nick Young.

The first Social Gloves event last year also had its problems ... with multiple fighters claiming they weren't paid.

McBroom told us last month he learned from the experience (the fighters eventually got paid) ... and promised a flawless event this time around.