A statement from Real, who beat rivals Barcelona to the La Liga title by five points this month, read: “After the Covid-19 tests carried out individually on our first football team yesterday by the Real Madrid Medical Services, our player Mariano has given a positive result.
“The player is in perfect health and complying with the sanitary isolation protocol at home.”
Real Madrid were beaten 2-1 by Man City in the first leg of the teams’ last-16 tie in February, before the coronavirus pandemic suspended the competition.
The second leg is set to go ahead at the Etihad Stadium on 7 August.
Diaz’s inclusion in his team’s squad for the match is likely to depend on negative test results for Covid-19 ahead of the trip to Manchester.