But speaking on the 'Inside Formula One' podcast for Unibet, the Finn has warned Sainz of the differences between the McLaren and Ferrari, saying that, while it may be all smiles now, the media attention in his new role will add weight to his shoulders.
"Knowing Carlos, he is a great guy, but Ferrari is a different team to McLaren. McLaren is pretty friendly, they are a nice team, you feel comfortable there," said Hakkinen.
"Going there [to McLaren], what he did with Lando Norris at McLaren was great, they matched together nicely, they had a good sense of humour, they brought a nice atmosphere to the team.
"Now when he goes to Ferrari, it's different. Now he has to show his full talent and potential otherwise it is going to be terrible for him. It will be so hard for everybody."
"When he enters he has to be mega professional in every aspect. Nobody is going to clean his mess. You have to be responsible yourself. That is going to be hard, because, in a team like Ferrari, you don't go there to learn, you have to be absolutely flat out. I don't just mean his lap time and speed at the track, but outside and between and understand the whole concept."
Hakkinen also pointed to the increased focus from specifically the Italian media, who can turn a driver from a hero to a zero overnight.
The Finn added: "What we have seen over the years how the communication through Ferrari and the media corresponds to the driver, it's not the easiest place to be unless you are winning. If you are winning, everything is very nice.
"Sebastian [Vettel], if you look at what's been going on the last couple of years, he has been criticised very heavily all the time.
"The self-confidence, what you need to have and the knowledge you need to have and what you demand, you need to know exactly what you do, so if any of those elements are lacking, you're going to be crucified there, which is normal."
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