Jose Gimenez has tested positive for coronavirus, Atletico Madrid have confirmed.
The Uruguay defender was the only player from Monday's tests to return a positive result, the club said in a statement on Tuesday.
Gimenez is now isolating at home, with Atletico confirming LaLiga's protocol has been implemented and the players will complete the next training session individually.
Head coach Diego Simeone also tested positive for the virus earlier this month and underwent a period of isolation, though he did not display any symptoms.
Atletico are due to start their new season at home to Granada at the Wanda Metropolitano on Sunday.
Simeone's men finished third in a disrupted 2019-20 campaign, while they were knocked out of the Champions League at the quarter-finals stage by RB Leipzig.