Newcastle striker Joelinton faces punishment from the troubled Premier League club after a social media post appeared to show the Brazilian breaching coronavirus lockdown rules.
Joelinton shared a picture of himself getting a haircut on Friday despite hairdressers being closed in England under the virus lockdown.
The 24-year-old deleted the tweet, but it was too late to avoid angering Newcastle.
"We are disappointed by the image shared by Joelinton," a Newcastle spokesman said.
"There are clear Covid-19 protocols in place and the club continues to guide and educate its players on their responsibilities. The club will be taking appropriate action internally."
It is the latest blow for Joelinton, who has scored just one Premier League goal this season and has been a flop since his move from Hoffenheim in 2019.
A Covid-19 outbreak last month forced the postponement of Newcastle's Premier League game against Aston Villa.
The club's training ground was also closed for a week after several players and members of staff having contracted the virus, including Allan Saint-Maximin and captain Jamaal Lascelles.
Earlier this month, Newcastle decided Saint-Maximin had not breached any self-isolation protocols after being picture on social media in a supermarket with a fan.
Joelinton's gaffe comes at a time when Newcastle boss Steve Bruce is under intense pressure.
Bruce's side are sliding down the league after a nine-match winless run, including five successive defeats.