The 24-year-old moved back to the Bernabeu last summer having spent two years with Juventus in Serie A and scored 15 La Liga goals from 26 outings.
However he had to live in the shadow of Cristiano Ronaldo, Karim Benzema and Gareth Bale, although the latter spent a large part of the season in the treatment room.
The Spain international opted to throw his lot in with the Blues and has scored three goals from four Premier League outings to announce himself to the Stamford Bridge faithful.
Despite calls to remain in Madrid, Mortata felt he had no choice but to leave for the sake of his career.
"Zidane wanted me to stay and I was happy in Madrid. But I couldn't stay to be a substitute," he told the Daily Mail. "You reach a point that you need to play, to grow up, to escape a comfort zone. It's about being comfortable. I don't want that. I am ambitious and hungry to succeed.
"For Spain, the more I play, the more goals I score, the more chance I have of making the World Cup and starting games."