Bayern Munich defender Noussair Mazraoui faces a period on the sidelines after medical tests showed up an inflammation issue relating to his heart.
The Morocco international tested positive for COVID-19 at the World Cup, Bayern said on Friday.
He returned to the team for the Qatar 2022 semi-final against France after missing the last-eight clash with Portugal, but subsequent tests have shown Mazraoui has a mild inflammation of the pericardium, which is the fluid-filled fibrous sac surrounding the heart.
Bayern travelled without him on Friday to a training camp in Doha, and the club have not specified a timeframe for Mazraoui's absence from the squad.
Newspaper Bild has reported the 25-year-old former Ajax player could be out of action for four to six weeks. That would make Mazraoui a doubt for the February 14 first leg of Bayern's Champions League last-16 tie against Paris Saint-Germain.
The Bundesliga leaders said in a statement on their website: "Noussair Mazraoui was not able to fly with the Bayern team to Doha on Friday as they prepare for the second half of the season at their training camp.
"The 25-year-old Morocco international tested positive for COVID-19 during the World Cup in December, meaning he missed the quarter-final against Portugal.
"During a follow-up examination in Munich after the World Cup, FC Bayern team doctor Professor Dr Roland Schmidt diagnosed a mild inflammation of the pericardium, which the defender will now cure.
"This means that Mazraoui will not be available to coach Julian Nagelsmann for the time being."
Bayern signed up Dutch defender Daley Blind on Thursday to fortify their squad ahead of the resumption of the Bundesliga after its extended winter break.
The 10-in-a-row German champions return to domestic action against RB Leipzig on January 20.
Midfielder Marcel Sabitzer also missed the beginning of the training camp due to flu, with plans for him to make the trip to Qatar once well.