Newcastle will take action against Joelinton over a social media post that appeared to show the player breaching lockdown rules.
The Brazilian posted a picture of himself getting a haircut on Friday, despite barber shops not being allowed to open under current lockdown rules.
Joelinton quickly deleted the tweet, but a club spokesman said: "We are disappointed by the image shared by Joelinton.
"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."
Awareness of the impact of the illness is acute at the club after an outbreak last month forced the postponement of the Magpies' game against Aston Villa.
The club's training ground was also closed for a week with several players and members of staff having contracted the virus, including Allan Saint-Maximin and captain Jamaal Lascelles.
Earlier this month, the club determined that Saint-Maximin had not breached any self-isolation protocols after being picture on social media in a supermarket with a fan.
