Arsenal is the strong favorite to end its nine-year trophy drought on Saturday when it faces Hull City in the FA Cup Final at Wembley.
But Lehmann believes the Arsenal "brand" has been seriously weakened by a lack of trophies and says he is "not sure" whether Wenger is still the man to restore Arsenal to the heady days of 2004, when the club went an entire season unbeaten.
Lehmann was the goalkeeper in that side, and he is the first of the 'Invincibles' to break rank and publicly question Wenger's future at the club.
“Is Arsene still the man to make Arsenal successful? I'm not sure,” Lehmann told Goal. “I think in the current conditions it will be very hard for them to win something. They have to make changes.
“If you have to do that always by yourself I doubt you can be successful.
“I think [chief executive] Ivan Gazidis is doing a great job there. But Gazidis came in when the times had changed a little bit.
“David Dein was supporting Arsene in terms of football, his valuation of players. At the time Arsene had the advantage of knowing the French market fantastically well.
“The markets have changed – the supply chain has changed. A club like Arsenal, who is majorly dependent on Arsene's view on football and philosophy and detection of new players, that is not enough any more.
“He is doing great but he can't do everything. A little bit of support on the sporting side in terms of football knowledge outside the training pitch, in my opinion could be helpful.
“Do I want him to stay but with a sporting director alongside him? Yes. Somebody like that can be there as support.”
Lehmann, who made over 200 appearances for the club, also believes Arsenal's standing in world football has been hugely damaged by their long run without a trophy.
“The FA Cup final is very important,” he added. “It's important for the club, the manager and the players. They have to win.
“The Arsenal brand is still a good brand but it isn't considered to be among the best football brands in the world any more.
“When I was playing there it was one of the two or three top brands in the world. It is a little disappointing it is not there any more. They have to start winning something and that will start on Saturday.”
Lehmann has also taken aim at Mesut Ozil, telling the club record signing he must improve his fitness to be considered 'world-class'.
Ozil, who cost 52 million euros from Real Madrid last summer, has played 39 games for his new club this season but missed almost six weeks with a hamstring injury suffered against Bayern Munich in March.
And Lehmann also insists he does not care Arsenal's squad now contains five Germans, claiming it doesn't matter who they are if they don't win anything.
“You are a world-class player first when you play most of the games,” said Lehmann when asked about Ozil. “You can't be a world-class player when you aren't available.
“Am I happy there is a German influence at the club now? I would be more pleased if they won the championship this year.
“No matter if they were German or not. They could have won it, they haven't won it – again. So I am not pleased. And when you don't win it more is needed – in every department.”