Mika Hakkinen says he is "quite surprised" that Fernando Alonso is already talking about retirement while facing a "massive task" with McLaren.
Joining McLaren at the start of this season, Alonso has made it clear that the Woking team will be his last in Formula 1.
Earlier this month the Spaniard, who is now in his 15th season as a F1 driver, stated: "After 15 or 16 or 17 years of Formula 1 – whatever it will be – it's enough.
"I started with McLaren-Honda when I was three years old – a replica of my father; I will finish with McLaren-Honda, but the real one in Formula 1, and that will be one third of my life."
His comments have worried former McLaren World champion Hakkinen.
The Finn, winner of the 1998 and 1999 Drivers' Championship, revealed he's concerned that Alonso may lose the motivation to improve McLaren if he's already talking about retirement.
Asked if winning should be the only thing on a drivers' mind, Hakkinen told the Daily Express: "You are right, if you start talking about things more important than winning or this is more important than racing, then something is not matching.
"I have not spoken to Fernando Alonso personally about that kind of subject but I was quite surprised how it was written that he was already planning retirement.
"He has taken a massive task to go with the brand new engine manufacturer and go to this development problem which is going to take a long time to get back to the top.
"McLaren has definitely great resources and a great team generally and great finances and I am very confident they can reach great results in the future – but it can take a long time.
"So you are right, is Fernando ready to work next two, three, maybe four years in a situation when you are not close to winning?
"It can be a really boring process because it's not only 15, 16 races in the season, it's a long year, travelling around the world, it's really, really tough, so can he motivate himself all the time?"
Added to that, Alonso also has to deal with the pressure of the world always watching over his shoulder.
Hakkinen added: "The world does not let him work in peace. Everybody is going to pressurise him to the maximum, saying, 'What's going on here? Come on!'.
"Because that's where the line is, all the time we keep pushing people to win and it will take a lot of patience and a lot of nerves from him to achieve being able to return to winning ways one day."