Castro and Maradona forged a close friendship since the football legend visited Cuba in 1987.
"I feel like the world has lost its leader," Maradona told Cuban state television after landing late Thursday.
"There can be a lot of players today, but he was the owner of the world team, because in South America today, or in Europe if you like, there is no leader as charismatic or capable of resolving things," he said.
Castro - who died on November 25 at age 90 - and Maradona forged a close friendship since the former Boca Juniors, Barcelona and Napoli legend visited Cuba in 1987.
Maradona temporarily moved to Cuba in 2004 to be treated at a hospital for a cocaine addiction and an array of health problems.
"Fidel is the one who gave me advice, who talked to me frankly, who told me what I had to do, what I couldn't do," said Maradona, who has the late bearded leader's face tattooed on his left leg.
He said he felt Cuban because of "all the love that the people" have given him during his multiple visits to the Caribbean island.
"He was the greatest, without a doubt. But now his legend must survive in our hearts," the retired World Cup winner said. "Fidel is not dead."
Cubans are observing a nine-day period of national mourning while Castro's ashes are being taken by road to the eastern city of Santiago de Cuba.
A ceremony is planned on Saturday night in Santiago with guest foreign leaders. The ashes will be laid to rest on Sunday at the cemetery where 19th century independence hero Jose Marti is buried.