Everton said they were "appalled" after images emerged appearing to show striker Moise Kean at a house party.
The Daily Star Sunday reported that Italian Kean, 20, filmed himself at the party in his apartment.
Everton said they were "appalled to learn of an incident in which a first-team player ignored government guidance and club policy in relation to the coronavirus crisis".
The club has "strongly expressed its disappointment to the player".
In a statement, Everton added: "Everton has regularly stressed the importance of following all the government guidelines - including rules and advice for inside and outside of the home - through a series of official communications to all staff members, including players.
"The amazing people in the NHS deserve the utmost respect for their hard work and sacrifice. The best way to show them respect is by doing everything we can to protect them."
As of Saturday evening, more than 20,000 people have died with coronavirus in UK hospitals, according to the Department of Health.
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Kean has had a disappointing first season at Everton following a £25.1m summer move from Juventus, having scored only one Premier League goal.
The striker was dropped from the squad for a game at Southampton in November by then manager Marco Silva as a disciplinary measure after being late for a team meeting for the second time.
Kean is the latest Premier League player to breach lockdown guidelines.
Aston Villa captain Jack Grealish was pictured at the scene of a car accident, after going to "see a friend". He apologised and said he was "deeply embarrassed".
Kyle Walker is waiting on any disciplinary action from Manchester City after reportedly breaking lockdown rules by hosting a party involving two sex workers.
Tottenham's Serge Aurier and Moussa Sissoko apologised for training together despite the coronavirus restrictions. And manager Jose Mourinho "accepted his actions were not in line with protocol" after being pictured holding a one-on-one training session in a park with midfielder Tanguy Ndombele.
And Chelsea midfielder Mason Mount was pictured having a kickabout with West Ham's Declan Rice despite team-mate Callum Hudson-Odoi testing positive for coronavirus and Mount being told to self-isolate for 14 days.