The 18-year-old was signed by the Gunners last summer and then loaned immediately back to Saint Etienne.
He told Bet Pensylvania: "William Saliba will start from the get-go when he joins Arsenal next season. He's currently playing for a struggling Saint-Etienne side, but his performances have been good, and he's been consistent.
"Defensively he's strong so he'll be looking to impress in pre-season. He'll need to adapt in that period, but he has enough time to learn from the Arsenal staff and players. He's still very young but has great experience from his time in Ligue 1.
"He'll continue to learn from the current centre-backs, but he'll have a big part to play from day one."