Madrid's top summer target is believed to be Monaco youngster Mbappe, who has scored 19 goals in 32 games this season.
The 18-year-old is also wanted by Manchester United, who have eyed the teenager as an alternative to Antoine Griezmann.
But Benzema, who left Lyon as a 21-year-old to join Madrid in 2009, believes Mbappe should wait before following in his footsteps by making a move abroad.
Karim Benzema claims Real Madrid and Man United target Kylian Mbappe shouldn't rush into a move
"I've been told he is a phenomenon, a very good player," he told French outlet RMC.
"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.
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Karim Benzema has established himself as one of Europe's top strikers at Real Madrid
"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?"