The Premier League is investigating whether there was a breach of transfer regulations after West Brom's Robert Snodgrass did not play against West Ham, according to the BBC.
The 33-year-old midfielder joined from the Hammers on January 8 and made his debut for Sam Allardyce's side in a 3-2 victory over Wolverhampton Wanderers on Saturday.
However, Allardyce said the player was unavailable for the trip to east London because of an "agreement between the clubs".
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"We could only get the deal done with that agreement," he said before the match at London Stadium.
Premier League rules state: "No club shall enter into a contract which enables any other party to that contract to acquire the ability materially to influence its policies or the performance of its teams in league matches."
Following the defeat, Allardyce told BT Sport: “If the Premier League want to know about it they can contact us privately and we will tell them.
“It is up to the Premier League to let us know if we have done anything wrong and then I will answer whatever questions they decide to ask me.
“It is not my big worry at the moment what the Premier League do or don’t think.”
Additional reporting from Reuters.
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