The centre-forward left Madrid for Lyon last summer, and has scored 14 times in 24 appearances for the French outfit during the 2017-18 campaign.
Los Blancos are believed to be closely monitoring the development of their former player, and Mariano has refused to rule out returning to the Bernabeu.
"I would not rule out a future return to the club, why not?" Diaz told Marca. "This is not a decision for me but for all the clubs involved and it is not an immediate issue, but yes one day it would be nice to return to Madrid.
"Right now I am happy at Lyon and the future is not at all on my mind, but hypothetically yes."
The 24-year-old scored five times in 14 first-team appearances for Real during the 2016-17 season.