Felix cut ties with Atletico last month for Chelsea.
Carrasco won a LaLiga with Felix at Atletico and told AS: "Everyone in football knows that João is a very talented player. He was born with a lot of talent, but it is clear that in football sometimes talent is not the most important thing, it is also discipline, concentration and work.
"His attitude was good, but he may not have done what the coach asked of him. When you are young, all attackers want to attack and not defend, and sometimes you may not have defended enough, as the coach wanted.
"Atletico is not such an easy team, because it requires a lot of effort there, and not just playing with talent.
"It was also difficult for me when I arrived young at Atlético, from Monaco. I was three months without playing. Then I earned my place, I had to change the chip in my head. There are players who cost a little more, it depends on each player, I was lucky that it clicked in my head, because otherwise after six months I would have left."
Carrasco added: " I wish him all the best and he will continue to grow year after year, because arriving at such a big club like Atlético at such a young age, with the pressure of such an expensive transfer, is not easy.
"I think he has improved and I hope he is happy at Chelsea and can show it to the whole world."