Saliba joined the Gunners from Saint-Etienne last summer, but was immediately loaned back to the Ligue 1 club for the 2019/20 campaign.
And former France international Sagna believes the 19-year-old centre-back will be a success in the Premier League.
"First he will need to adapt to English football," the former Gunners star told Goal. "He is a big prospect for the future but Arsenal need him to perform as soon as he starts.
"He is aware of that probably, but he has a lot of qualities and he will learn fast. Hopefully he will achieve good things and become one of the best centre-backs in the league.
"He is very confident in defending, he is not afraid to defend one against one and most of the time he wins the duels, which is the best quality as a defender.
"You need some security, you need some presence and he has that."
And Sagna dismissed any concerns about Saliba's age.
"Someone who has always said don't speak about my age is Kylian Mbappe," add the 37-year-old.
"When he started saying this he was 19 and now he's one of the best strikers in the world.
"If he [Saliba] believes he can be as good as Van Dijk, he's probably been watching his games and analysing them. It's about hard work.
"If you want to be the best at the back, it's all about work and belief."