The 29-year-old right-back came through Real's youth system and made six La Liga appearances before leaving the Bernabeu in 2006.
However, he is now set to be part of the Atletico Madrid team that will seek to defeat their city rivals in Lisbon on Saturday.
Ahead of the Champions League final, he told UEFA.com: "I'm very grateful to (Real) Madrid for the years I spent there, how they treated me and trained me, but now I'm an Atletico Madrid player and I am aware I've been wearing the best colours.
"Of course, matches against Real Madrid are different, they are more special, but all I want and all I think about, above all, is victory for Atletico - the past is behind me."
However, winning Europe's premier club competition for the first time would represent an even more remarkable achievement.
"It would be incredible, for me and my family," added Juanfran.
"They are proud of what I have done and what I am doing, but, of course, being a European champion and being the best team in Europe is as good as it gets."
