The forward is on the brink of signing for Sporting CP.
Former Racing Club vice-president Pablo Podesta told A Bola that Sporting must be wary of who they're signing.
He claimed: "He is very good at deciding and dribbling, although he is not very strong physically, he is intelligent, he understands the game, he is skillful and he has a good technique.
"But when he has a player in front of him he has difficulties trying to get rid of him."
He added: "There's some problems in terms of character and personality, which is probably why (Diego) Simeone hasn't used him for Atletico Madrid."