Diego Simeone has tested positive for coronavirus, Atletico Madrid have confirmed.
The 50-year-old Argentine is not displaying any symptoms, and will self-isolate at home in line with social distancing guidelines.
Simeone had been back in training with his side since last Monday ahead of the new La Liga season.
Atletico are scheduled to play Cadiz in a friendly game on Tuesday, ahead of their season opener at home to Granada on September 27.
A statement from the club said: "The first team, coaching staff and auxiliary personnel underwent tests on Friday as soon as they returned from training in Los Angeles de San Rafael after a positive test was returned by a member of the travelling party in the previous tests carried out on Thursday.
"The analysis of these new samples in the laboratory has determined that our coach, Diego Pablo Simeone, has given a positive result for Covid-19.
"Fortunately, he does not present any symptoms and is isolating at his home and fulfilling the quarantine after taking training with the team since last Monday."
Simeone is not the only leading figure to test positive for Covid-19 since clubs in Spain returned from the summer break.
At the beginning of September, striker Diego Costa and right-back Santiago Arias were forced to delay their return to training after both returned positive tests.
Both players were also asymptomatic, with the pair also sent home to self-isolate.
Despite the general success of the return to football after lockdown, there are concerns once more over welfare as players return from summer holidays.
Paris Saint-Germain lost the opening game of their Ligue 1 season 1-0 at minnows Lens after a number of star names, including the front four of Neymar, Kylian Mbappe, Mauro Icardi and Angel Di Maria, were ruled out with the virus.
Paul Pogba missed Manchester United's final pre-season friendly against Aston Villa on Saturday, after the latest round of testing in the Premier League returned three positive cases.