Wenger has faced unprecedented criticism from some Arsenal fans during a summer that has brought the departures of key midfielders Cesc Fabregas and Samir Nasri, with criticism focused on the Frenchman's perceived failure to address his team's weaknesses in the transfer market.
Arsenal visit Old Trafford on Sunday with only one point from their first two Premier League outings, a goalless draw with Newcastle and a 2-0 home loss to Liverpool.
However, they did lift some of the gloom by beating Udinese to qualify for the Champions League group stage on Wednesday.
Defending Wenger, Ferguson described him as "the best" manager in his club's history. He told reporters: "I'd like to know who's going to replace him - the work he has done for the 15 years he has been at the club is the best in Arsenal's history."
Ferguson argued that supporter culture in England had changed, with fans less willing to tolerate lean spells than in the past.
"Who understands fairness and unfairness today? It is quite a cynical world and supporters are far less easy to please than 20 years ago," he said.
"It is disappointing but they answered everyone back on Wednesday, his team played well, had a great result. A lot of people thought they would be out I didn't think so, I thought they'd win it."
Arsenal have not won a trophy since beating United in the 2005 FA Cup final, and the last of their three league titles under Wenger came in 2004. Between 1996 - when Wenger arrived in North London - and 2004 United and Arsenal were the only two sides to lift the Premier League trophy, leading to an intensive rivalry between the pair.
Ferguson said the spending of first Chelsea and Manchester City has broken that duopoly and made it more difficult to win the big prizes.
He said: "[Wenger] has not won a trophy in six years but what does that mean? The quality of his side has not been reduced, he's still got great quality in his teams in that period, when you look at the competition it has stepped up in the last few years.
"Chelsea's involvement has created a lot of that problem in terms of trophies to win because they and ourselves have been dominating the Premier League and now you've got Man City who've come on the scene, and Tottenham had a great season last year and the season before."
Ferguson added: "I don't know the reasons [for Wenger's lack of spending], some clubs have financial constrictions - that may be the reason. I don't know the full story about Arsenal's finances."
