Moyes succeeded Ferguson last summer and has found the task challenging, with the team that won the Barclays Premier League last May unable to mount a sustained challenge this term, even for the Champions League places.
But the former Everton boss has suggested any manager, even Ferguson, may have struggled with the squad he took on, given the age of many of the players.
United's squad includes a number of players aged over 30, including centre-backs Nemanja Vidic and Rio Ferdinand, left-back Patrice Evra, midfielders Ryan Giggs and Michael Carrick, and striker Robin van Persie.
"People are aware there is a squad that is a bit more ageing, so I think it would have been a tough season for whoever was in charge of Manchester United this year," Moyes told national newspapers.
"And I actually think if Sir Alex was here this year it would be difficult for Sir Alex as well and I'm sure he would be aware of it.
"It could have been the case no matter what this season, but it has been so un-Man United, which is why we have to look to continue a policy of building, improving, getting better and that's what we'll try and do."
Jose Mourinho leapt to the defence of Ferguson, however.
Mourinho, who this term has repeatedly rebuffed suggestions he was a potential successor to Ferguson, said: "Lots of people are speaking about Sir Alex. He made Man United what Man United are.
"He decided to retire. When somebody decides to retire, especially a person like him, he deserves to retire. He deserves to enjoy his life.
"And now he's responsible for what? Man United lose a match it's his responsibility? Man United struggling in the Premier League is his problem?
"(Wayne) Rooney misses a goal, it's his fault? Or (David) de Gea makes a mistake and it's his fault?
"He can only be blamed for one thing: Man United being the club Man United are. He's retired. Let him retire in peace."
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