The incident came after Man U's loss to Everton.
Merseyside Police are investigating an alleged assault after Manchester United forward Cristiano Ronaldo appeared to break a fan's phone.
Footage was shared on social media alongside comments claiming he smashed the device as he left the pitch after his side's 1-0 loss at Everton.
Ronaldo, 37, later apologized on social media.
Merseyside Police said they were investigating "reports of an alleged assault" at Goodison Park on Saturday.
A spokesperson said they were working with both Manchester United and Everton.
"As players were leaving the pitch at 2.30pm, it was reported that a boy was assaulted by one of the away team as they left the pitch. Inquiries are under way and officers are currently working with Everton to review CCTV footage and are carrying out extensive witness enquiries to establish if an offence has taken place."
Manchester United said they were aware of "an alleged incident" and would "co-operate with any police inquiries".
The result damaged United's hopes of qualifying for the Champions League and came against an Everton side who had been one point above the relegation zone before kick-off.