The 34-year-old, now back at boyhood club Boca Juniors in Argentina, has trained and played with both stars.
Tevez was team-mates with Ronaldo at Manchester Untied and has represented Argentina alongside Messi.
Explaining what both do behind closed doors in training, it’s clear to see there’s a major difference.
“Lionel Messi, I never saw him in the gym,” he told the Argentina media. “I’ve never seen him train in the stops or do technical exercises.
“It is all natural to him, but on penalties he trained: first he did not take them and now he shoots them all at the corner.”
On former United team-mate Ronaldo, he revealed: “Cristiano always stopped in the gym after training: for him it is an obsession and, being the best in everything, he always arrived at work early.
“When the training was set for 9 in the morning I arrived at 8 and he was already there.
“Even if I arrived at 7.30 he was already there.
“I began to ask myself, ‘How can I get rid of this guy?’ So one day I arrived at 6 but he was already there! Sleepy, but he was there.”