Nico Rosberg believes Michael Schumacher will find it tough to make a convincing comeback to Formula One after four years in retirement.

Rosberg has already been signed by Mercedes, who have taken over Brawn GP, for the 2010 season and rumours are growing that German compatriot Schumacher will be unveiled as his team-mate.

Speculation has been fuelled by the fact that, while the signing has not been confirmed, neither Mercedes GP nor Schumacher have denied the rumours.

Schumacher, who will turn 41 on January 3, retired from Formula One in 2006, and 24-year-old Rosberg believes he will find it difficult to return.

"It will not be easy for him," Rosberg told the Stuttgarter Zeitung. "When you stay away from Formula One for a long time, it takes time to get used to all the trial sessions again, it is not simple."