De Luis had so impressed at Rayo Vallecano this term that he was close to being promoted to the senior team under Paco Jemez once football resumed after the COVID-19 pandemic.
The player himself made his farewell to the club official on his social media pages on Tuesday afternoon.
"Today is a difficult day for me," he posted.
"I have to say goodbye to what has been my home for the past four seasons.
"There are many memories, emotions and the images of everything I've lived with this club, Rayo has made me grow in every way and has given me values that I will always hold."