The former France midfielder, who spent three years at Highbury, told the Mirror: "How long will it take for these young players to be ready to be really challenging for the league?
"I don't know - if they become very good players Arsenal are going to sell them. It's good sometimes to sell top players when they get to 30 or 31, but when I have a look at the Arsenal politics in the last five years we sold our best players: Pires, Vieira, Henry."
Petit, now an analyst for sports channel Setanta, added: "If you sell a player and you get money it's fair enough but you have to realise what you're losing when you sell a player.
"If you sell players who represent the image and the spirit of the club, for example Patrick Vieira, you can spend a lot of money, you can find very good players, but in the dressing room and on the pitch you need leaders.
"I think this year is very important for Arsenal. If they don't perform well and don't finish in the top four then financially they will be struggling and there will be less money on the transfer market.
"It becomes a downward spiral - unless Kroenke or somebody buys the club.
"I just want the board to realise that if you want to win titles you need the dynamism of the young players, but the experience and maturity of the older players as well.
"The last two years they finished one or two points off the Champions League and maybe think now is the time to spend to go a step forward. I think they can get a Champions League spot."