"Messi doesn't need to win the World Cup to be the best player in the world," Maradona, who inspired Argentina to the title in Mexico in 1986, told La Nacion.
"(If he did) it would be great for Argentina, the fans and Lio, but (winning) a World Cup or not won't take away any of his achievements up to now to be among the greatest."
Twice World Cup winners Argentina face Romania in a friendly in Bucharest on Wednesday as they begin to crank up their preparations for this year's finals in Brazil.
Former captain and coach Maradona said everything appeared in place for Messi to shine although Argentina's task would be tough with hosts Brazil, title holders Spain and Germany among the sides to beat.
Maradona said Messi had had a great World Cup in South Africa in 2010 despite not scoring any goals. The Argentina team coached by Maradona lost 4-0 to Germany in the quarter-finals.
"Messi had an exceptional World Cup with me... and no-one said so. (Was it) because he didn't score? He turned all the goalkeepers into stars," he said.
Maradona said he could not forget Messi's sobs when Argentina were eliminated.
"I approached him and told him he'd have many World Cups to get revenge. I said it with all my heart. While the rest were thinking about our return (home) he was there, head bowed, crying."
Argentina are in Group F with Bosnia, Iran and Nigeria at the finals starting on June 12.
"There's Spain, Germany and there's Brazil with a tremendous defence. You think of Brazil's defence and six or seven names come to mind, you think of their attack and there's only Neymar," Maradona said.
"For Lio it will be a test of character, to bring out all that crying he has in his heart. Brazil can be his great revenge," added Maradona, who will commentate on the tournament for a Venezuelan television programme.
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