Mariano, 25, debuted for Madrid's first team at age 23, after struggling for a place for several years, though is set to start the derby after returning to 'Los Blancos'.
Before leaving for France, the striker had played just six minutes in the Champions League and 109 in La Liga for Real. In the Copa del Rey he had featured for 183 minutes; the lack of first-team opportunity was the main reason behind his exit.
However, Mariano was welcomed back to the capital after scoring 21 goals and assisting 6 more at Lyon last season.
The academy-product shined as he scored the goal of the game against Roma after coming on as a substitute, and since then, Lopetegui has rewarded him with half-hour cameos against Espanyol and Sevilla.
Mariano has transformed as a player, and has already acheived more under the Basque manager than under his predecessor Zinedine Zidane. This, coupled with the goalscoring draught of Benzema, suggests the Spanish-Dominican striker will start against Atlético on Saturday.