Portugal international Joao Felix is self-isolating at home, as team-mates Mario Hermoso and Yannick Carrasco have been doing since Saturday (AEDT).
"Joao Felix has tested positive for COVID-19. The striker is isolating at home, in compliance with the health authorities' guidelines and LaLiga protocols," an Atleti statement read.
Simeone's men have a 10-point advantage at the top of LaLiga and welcome Celta to the Wanda Metropolitano on Tuesday (AEDT). They then travel to Granada the following Sunday (AEDT).
Hermoso and Carrasco are unlikely to return for the meeting with Celta, while Kieran Trippier will be absent due to suspension.
Joao Felix marked his return to the starting line-up against Valencia on January 24 by scoring his first goal in almost two months and setting up another, leading Atletico from a goal down to a 3-1 victory.
The 21 year-old has been left out of the first XI eight times in LaLiga this season and Atletico has won all those matches.
However, when Joao Felix starts they average two more shots (12.1 to 10.0) and their goals-per-game ratio improves from 1.9 to 2.3.