Karim Benzema sees Kylian Mbappe as a phenomenon, but has warned against him leaving Monaco so early in his career.
Benzema is familiar with this situation, as he was a young star with Lyon, before leaving for Spain.
Mbappe has been a revelation for Monaco and is being courted by top clubs in Europe, with Madrid and United among the clubs interested in his signature.
Benzema told RMC: “I’ve been told he is a phenomenon, a very good player.
“Honestly, I still haven’t seen him. He’s a young player.
“Football is really hard, especially at big clubs. I arrived at Madrid at 21. I had already done a lot of things at Lyon, but when I got here I got a slap in the face.
“My first year was too difficult. You’re far from your family, it’s not the same. Those who are next to you are there to take your place. It’s very difficult.
“But from what people have said, he’s a very good player, and I hope he’ll have a great career, little by little.
“I think you mustn’t leave too soon. Things go well, but the day when it goes less well, that’s the problem.
“He’s 18, how do you handle it when it doesn’t go as well, when people don’t speak as well about you, when you’re put under pressure?”
Mbappe, who finds himself in the latest France senior squad, has netted 17 times in 29 appearances for Monaco this season.