The Bulgarian striker played for United from 2008 until 2012, featuring alongside Ronaldo during his final season before the Portuguese's world record £80 million transfer to Real Madrid.
During the 2008-09 season, Ronaldo scored 26 goals in 53 appearances for Alex Ferguson's side, with Berbatov's 14 in all competitions helping United win the Premier League and the Club World Cup.
Despite a limited time playing together, Berbatov was impressed with Ronaldo, and having watched him at Madrid, believes he is one of the best footballers ever.
“No doubt he’s one of the greats," Berbatov told BT Sport.
"The way he’s keeping himself [fit] and we know how he used to train. He wants to be the best, he is the best, one of the best."
Should Zinedine Zidane's side progress, they will play Juventus, with Berbatov projecting Ronaldo to have a major influence on that game's final outcome.
"No doubt in the final – because I think they will be in the final – he’ll be a big problem for Juventus," the 36-year-old asserted.