Everton midfielder Ross Barkley "was the victim of an unprovoked attack" in a bar on Sunday, lawyers acting on his behalf have said.
Merseyside Police have confirmed they have opened an investigation following the emergence of footage appearing to show the 23-year-old being punched.
The incident is alleged to have taken place in Liverpool city centre, some time after Barkley featured in Everton's 4-2 victory over Leicester on Sunday.
A statement from lawyers read: "We can confirm that Ross was the victim of an unprovoked attack by a stranger who approached him on Sunday evening."
Footage of the alleged incident has appeared online via social media. The matter has not been reported to police but the Merseyside force has decided to investigate.
A statement read: "Merseyside Police is aware of footage online reportedly showing an assault in a bar in Liverpool city centre.
"No report of an assault has been received but officers are making enquiries to establish the circumstances of the incident."
Barkley played 90 minutes against Leicester but his future was thrown into doubt as manager Ronald Koeman later said the player would have to be sold if he does not sign a new contract.
The Academy graduate's current deal expires at the end of next season.
Koeman said: "We offer him a new contract, and then (there are) two possibilities.
"One, he signs that contract. If he doesn't sign that contract then we need to sell the player."