Jenson Button has admitted that Sergio Perez could struggle when he makes the move to McLaren next season.
Perez will replace the Mercedes-bound Lewis Hamilton but Button admits that it will be a "massive ask" for the Mexican to fill his current teammate's seat.
"It's going to be a tricky seat for Checo to fill," Button said. "Lewis is a guy who has won the world championship and had so many wins and races with this team.
"It is a massive ask but what else can you do? You need to have someone in the car and he is quick.
"I've heard he's intelligent too. I don't know if he will be up to it straight away. He is going to be spending a lot of time in the United Kingdom at the factory."
Perez has seen his form for Sauber decline in recent weeks and has claimed only one point in the last six races.
