The guy who backs up Russell Wilson is being investigated by police for allegedly punching a man in the face at a bar on St. Patrick's Day ... TMZ Sports has learned.

25-year-old Alex McGough -- a 7th round pick in 2018 out of FIU -- was allegedly at the Green Parrot Pub in Tampa, FL when an altercation broke out ... and things got violent.

TMZ Sports spoke with the alleged victim, 24-year-old Anthony Albino -- who tells us he was reconnecting with an old friend at the bar when McGough approached him and accused him of shooting his shot at his girlfriend.

Anthony says the NFL player got heated -- and all of the sudden, McGough socked him in the face out of nowhere.

In video taken after the alleged incident -- and obtained by TMZ Sports -- you can see Anthony with red marks on his face ... surrounded by bloody paper towels he used to clean himself up.

Anthony says he suffered minor injuries from the incident ... including a bloody nose and a gash to his elbow.

We're told Anthony went to cops the following day and filled out a report with the intention of pressing charges.

So far, McGough has neither been arrested nor charged with a crime -- but our law enforcement sources confirm the incident is under investigation and officers are hoping to speak with McGough soon

We've reached out to McGough's camp for comment ... so far, no word back.

The Seahawks signed McGough to its practice squad back in December 2020 -- and he's currently listed as the #2 quarterback on the team depth chart.