The Football Association will look into Eberechi Eze's attendance at QPR's FA Cup match against Fulham on Saturday.
The Crystal Palace winger, 22, sat in the directors' box at his former club to watch their third-round tie.
With the country in lockdown and matches still being played behind closed doors, questions have been asked about whether coronavirus protocols were breached.
Eze was pictured not wearing a mask as he watched his old team slip to a 2-0 defeat.
The PA news agency understands the FA did not give any approval for Eze to attend the match, and that the governing body will be looking into the matter.
Eze is understood to have been a guest of QPR, the club he left for £20million to join Palace in the summer.
Rangers manager Mark Warburton was pressed on his attendance after the match but would only say: "I've no idea whether he broke protocols or not.
Statement from Luka Milivojevic — Crystal Palace F.C. (@CPFC) January 6, 2021
"You'd have to ask our health and safety officer because that's above my pay-grade.
"I want to talk about the game. What other players do in their own time, I can't comment on that. It's not my area of expertise. Far from it."
Palace have so far declined to comment, but manager Roy Hodgson had previously made it clear to his players that no further breaches of Government guidelines will be accepted after Luka Milivojevic broke the rules on New Year's Eve.
Palace captain Milivojevic issued an apology last week after his household mixed with Fulham forward Aleksandar Mitrovic and his family.
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