Inter have made a slow start to the season, but the 4-0 victory over Bari on Wednesday suggests Benitez may be getting to grips with life in Serie A.
After Mourinho led the side to success in the league, cup and Europe last season, though, Ferguson believes the former Liverpool boss faces an uphill battle.
"It's tough to take the place of someone who has won as much as Mourinho," he told Gazzetta dello Sport.
"One day whoever succeeds me at United will have the same handicap. In all cases, every coach who has respect for himself must give his own identity to the team.
"You can be certain that beating Inter will always be difficult. The European champions have all of my respect. I bet on Inter against Barcelona in the semi-final. Jose prepared for it like a champion, and what a phenomenon Cambiasso was on Messi."
Ferguson also hailed former Inter striker Mario Balotelli, who was linked with Manchester United this summer before joining Manchester City.
"Balotelli has extraordinary potential for a 20-year-old," he said. "It's a shame about his injury. We hope to see him in action soon."
He added: "[United striker Federico] Macheda is only 19 and he is doing magnificently. What a future for the Italian national team when they are both ready together. I am very satisfied with Chicco. He is simply fantastic."