Wenger has come under scrutiny after Arsenal failed to end their six-year search for silverware last season, the club having last claimed glory in the 2005 FA Cup final against the Red Devils.
The French coach has also come in for criticism for his transfer policy after both Cesc Fabregas and Samir Nasri were sold this summer, while Gervinho and kids Alex Oxlade-Chamberlain and Carl Jenkinson are Wenger's only additions to the squad so far.
But Ferguson, who locks horns with Wenger this weekend at Old Trafford, said: "I would like to know who is going to replace him.
"The work he has done in the 15 years he has been at the club is the best in Arsenal's history. Yes, he has not won a trophy for six years but what does that mean? The quality of his side has not been reduced.''