Kempes, a World Cup winner with Argentina in 1978, believes Messi has become more of an individual player in recent times at Barca, while Ronaldo is now more of a team player at Juve.
“Messi is Messi and Messi is Barcelona,” Kempes told newsmen.
“What you do or do not do is key. When Messi is on the field, prepare yourself, because you do not know at what moment he is going to mess you up.
“But Messi should not surprise us. We have seen him do everything, although I think he has slowed down a bit and now I see him wanting to do more things on his own.”
Kempes added: “Before I saw him as a team player, now I see him more selfish in his actions.
“It is the opposite of Cristiano, who now plays more for the team than for the ‘me’.
“I will tell you another thing, we Argentines criticise Messi because we are specialists in criticising what is ours.
“If you ask me who I would have liked to play with, I would say that with Messi … because I already played with [Diego] Maradona.”
Messi scored in Barcelona’s 2-1 shock defeat to Valencia in the Copa del Rey final on Saturday night.