Sebastian Vettel has admitted it is "difficult to judge" whether his friend Kimi Raikkonen will be at full speed when he returns to formula one next season.
The pair often meet near their Swiss homes to play badminton, but reigning world champion Vettel does not want to predict a smooth comeback for the 2007 world champion after two years in world rally.
"We have all seen Michael's comeback," the 24-year-old is quoted by France's 20minutes.fr on a visit to Red Bull's engine supplier Renault.
"He (Schumacher) left when he was at the top, but when he came back he said a lot had changed and that he needed time to adapt.
"If I think about the car I drove two years ago, so many things have changed," said Vettel. "It's amazing. Even during a season, new buttons appear on the steering wheel.
"I can imagine it would be very difficult to return straight away to the same level as before," added the German.
Renault/Lotus boss Eric Boullier revealed on Tuesday that Raikkonen will soon be able to use the team's new driver simulator, and get some laps under his belt in a two-year-old "demo car" before real testing begins in February.
Britain's Guardian wrote: "If he got fed up with driving a Ferrari ... what will happen at Renault, a midfield outfit who are unlikely to be challenging the top teams next year?"
Mercedes' Norbert Haug is quoted in German reports: "I look forward to seeing Kimi back if he is motivated, and that seems to be the case."