A statement said the 26-year-old Spanish-born player was in “perfect health” and “complying with the sanitary isolation protocol at home”.
He was an unused substitute for Real’s last match – a 2-2 draw in La Liga away at Leganes on 19 July.
Madrid play at Manchester City in the second leg of their last-16 Champions League tie on Friday, 7 August.
City hold a 2-1 lead after the Bernabeu first leg, which was played in February before the coronavirus pandemic caused the competition to be halted.
Mariano has made seven appearances for Real in all competitions in 2020 and scored in a 2-0 win over Barcelona at the beginning of March, a result that helped Real become Spanish champions.