Henry, who scored 175 goals for Arsenal, pointed out that the Gunners are in dire need of a goalkeeper, centre-back, midfielder and striker.
The 37-year-old's comments come after Arsenal's goalless draw against Chelsea on Sunday.
"Arsenal need to buy four players, they need that spine," he told Sky Sports. "They need a goalkeeper, they still need a centre-back, they still need a holding midfielder and, I'm afraid, they need a top, top-quality striker in order to win this league again.
"The best example for me today was Chelsea. They struggled last year and they were still competing to win the league. They now have [Thibaut] Courtois, [Nemanja] Matic, Cesc [Fabregas] and Diego Costa.
"For me that speaks volumes, and we saw it today. A team didn't and a team did, and Chelsea are about to win the league."
Henry also made it clear that if manager Arsene Wenger wants his side to become the team to beat, he will have to start increasing competition amongst his players.
"He has to [sign players]," Henry said. "If you take the goalkeepers, for example, at Bayern Munch, Chelsea, Manchester United and a lot of other teams.
"You need that type of player, you need that competitiveness within the group and you need great players. In order to win the league that has to happen."
Henry was also less than impressed with midfielder Mesut Ozil's performance against Chelsea.
"Players need to perform also," he said. "I will single out Mesut Ozil, who made his eighth appearance against a top-four team today.
"Zero goals, two assists. That is not enough. Since joining Arsenal he has 14 assists. Now if you look at Fabregas at Chelsea, he has 16 assists in one season.
"At one point, I do think, and I do believe, the players have to step up and stand their game up."