Before the resumption of training on Friday, the entire first-team squad underwent tests except for Costa and Arias.
“The team has been tested except for those involved in international fixtures with their countries,” said an Atletico statement.
“Diego Costa and Santiago Arias, who tested positive during their holidays, are currently isolating and show no symptoms.
“Both players are staying at home complying with all protocols and measures set by the authorities and La Liga.
“Therefore, they will not resume training on Sept. 4 and their return is expected after completing their quarantine.