The Belgian winger suffered a collarbone injury in Atleti's 1-0 defeat to Villarreal on April 25 and had not been expected to return in time for the trip to the Santiago Bernabeu on Tuesday.
But the 23-year-old has unexpectedly been passed fit to provide a boost for Diego Simeone, who is without injured defenders Jose Gimenez, Sime Vrsaljko and Juanfran.
There were no surprises in the Real Madrid squad, with Raphael Varane available to return in defence after a period on the sidelines with a hamstring injury.
Atleti midfielder Gabi believes his side must play a "perfect" game to defeat their rivals, who inflicted heartbreak upon them in the 2014 and 2016 finals.
"We have to control our emotions very well and play the perfect match to try to beat Real Madrid," Gabi told a pre-match press conference on Monday.
"In fixtures as evenly-matched as this one, any detail, any success against the opposite goal, are the ones that set the result."