Hampshire Police are investigating an incident between Cardiff's Krystian Bielik and a home supporter during Portsmouth's 2-0 win at Fratton Park on Saturday.
Bielik was involved in an altercation with a fan as he went to retrieve the ball in the second half. Police were notified of the incident and are investigating.
The Championship game was briefly halted around the 70-minute mark after Bielik, recently signed from West Brom, went to retrieve the ball from one of the stands.
"We can confirm that we are investigating an incident in which a teenage boy was reportedly the victim of a common assault during Saturday's football match at Fratton Park (5 September). Our enquiries are ongoing," a police statement said.
Following the match, Portsmouth urged supporters not to speculate on social media while the investigation is ongoing.
Video footage of the incident shows Bielik becoming involved in an altercation with the young male supporter, who caught the ball and then appeared to be pushed in the throat by the player after he had thrown it back towards the pitch.
Cardiff boss Brian Barry-Murphy said Bielik was slapped in the face, while his opposite number John Mousinho apologised to the visiting team.
A statement on portsmouthfc.co.uk on Sunday said: "Portsmouth Football Club are aware of supporter concerns regarding the incident involving a young fan and an opposition player at yesterday's match at Fratton Park.
"As this remains a safeguarding matter and is currently under investigation by both the club and the police, it would be inappropriate for us to comment further on the circumstances at this time.
"However, we are aware that a number of claims and assertions are circulating on social media. We would urge caution against speculation while enquiries are ongoing.
"The welfare and safeguarding of young people are of paramount importance to Portsmouth Football Club. We will continue to liaise with the relevant parties as enquiries progress."
Pompey fans on social media have called for Bielik to be punished, although Mousinho took the view after the match that the away side had been wronged.
He said in quotes reported by the BBC: "It wasn't a good incident. I have just seen him (Bielik) there, and he was OK, but that's not the point, we can't be having that.
"We want to create a really difficult atmosphere at Fratton Park, we have done that successfully over the past few years. Sometimes it tips over, I understand that, I don't think it needs to tip over to that extent.
"We extend our apologies to the players, Cardiff, and make sure we can try to cut it out."
