Felix left Atletico last month to move on-loan with Barcelona, where he has starred this past week.
In reaction, Saul said: "He is a great player that you have already seen that later he has had the bad luck of not being able to demonstrate it in this team.
"I can't tell you anything else because I already tell you, as a player he is very good, very very good.
"Then ... You have to want, you have to adapt, you have to try to do many things that maybe if your head was not at the best time, because he could not do it here."
Saul added, "I wish him all the best, that everything goes very well in Barça. He has started very strong, I think he could have done things better too, he has not done it here, and I think that in the club and in the team it has taken its toll."