Atletico Madrid have confirmed that Lucas Torreira has tested positive for coronavirus after returning from international duty.
The midfielder has no symptoms but is now isolating at home and is a doubt for Saturday's blockbuster La Liga clash with Barcelona.
Torreira had been on international duty with Uruguay, starting both his country's defeat to Brazil on Tuesday and victory over Colombia on Friday.
Atletico announced the news on the official club website, releasing a statement which read: "Today, Lucas Torreira underwent PCR tests upon his arrival in Madrid after returning from his commitments with the Uruguayan team following the usual protocol when our internationals rejoin after their commitments with their country.
"The analysis of these samples has been positive for COVID-19. Our player has no symptoms and remains isolated at home, complying with the corresponding quarantine, following the protocols of the health authorities."
The Arsenal loanee joined Atletico on deadline day, with Thomas Partey moving in the opposite direction. He has made seven appearances in all competitions since moving to the Spanish capital - although five of these have been substitute cameos - scoring one goal.
Atletico have enjoyed a thoroughly impressive start to the season and are currently unbeaten in La Liga, winning five and drawing two of their opening seven matches.
Victory on Saturday could see Diego Simeone's side go top of the table if Villarreal fail to beat Real Madrid in the day's earlier kick off.
However, not only is Torreira now a doubt for Saturday's clash, Atletico will be without his compatriot Luis Suarez, who has also tested positive for COVID-19.
Suarez was due to face his former side for the first time since swapping Barcelona for Atletico in the summer, but has returned two positive coronavirus tests while on international duty and is currently quarantining.