F1 witnessed a "different" Sebastian Vettel in 2018.
That is the view of Massimo Rivola, a former top Ferrari engineer who also worked with the German driver in his Toro Rosso days.
Vettel looked the favourite to win the world championship last year until a mid-season slump.
"I don't know what happened," Rivola told La Gazzetta dello Sport.
"Every athlete needs the perfect balance to be at his best. I don't know the Vettel of 2018 -- my Vettel was different.
"I only hope that (Charles) Leclerc stays as he is," he added, referring to the young star who has been promoted by Ferrari to be Vettel's teammate from 2019.
Leclerc and Vettel will have a new team boss this year in the form of Mattia Binotto, and Rivola said: "I hope Mattia can work with serenity, but that's not easy at Maranello.
"He certainly has all the qualities to succeed."
Rivola has now left F1 altogether, instead turning to the world of MotoGP with Aprilia. "It's never easy to leave Ferrari," he said, "but Aprilia is a nice new challenge."
