The 51-year-old, who took over at Old Trafford in July 2013, was sacked in April with United seventh in the league and no longer able to qualify for the Champions League.
But speaking to an English daily on Sunday, Moyes said it was difficult to replace Ferguson, who enjoyed a trophy-laden 26 years in Manchester before retiring.
"It was a step into the unknown and, looking back now, it was near enough the impossible job," Moyes said.

"I was devastated to lose the job because it was something I felt we could make a real success of. We knew it was going to take time to make the necessary changes. It was going to take time to evolve. But we were in the process of making other important changes,” he added.
"In the end, I don't feel I was given time to succeed or fail,” he said.
His replacement, Dutchman Louis van Gaal, lost his Premier League opener 2-1 at home to Swansea City on Saturday with pundits expecting another tough season if more quality is not brought in during the last two weeks of the August window.
Moyes dismissed suggestions he had fallen out with fellow Scotsman Ferguson after his sacking but did admit the club failed to secure their top transfer targets during his time in charge.
At the time of Moyes' arrival at United, Ed Woodward replaced David Gill as executive vice-chairman and the former Everton boss said a lot had changed at the club in a short space of time.
"We tried to bring players in during the summer transfer window but they didn't materialise," he said.

"It certainly wasn't for the lack of trying. It wasn't indecision. The complete opposite. It's been well documented that we wanted (Cesc) Fabregas, (Gareth) Bale and (Cristiano) Ronaldo. There was talk of Ronaldo when I first arrived,” he added.
"I had taken over from the most successful manager in history. The chief executive had taken over from one of the most renowned administrators in the game. So it was a new job for two people,” he further said.
Moyes, who spent 11 trophyless but successful years in charge of Everton before heading to United, believes he has nothing to prove as a manager and may move abroad for his next job.
"I've been sounded out by a few people about different jobs but I'd like to find a job, if I can, that gives me a chance of being successful," he said.
"I worked long and hard to get to that level. I put in an awful lot of effort for it to be taken away from me,” he added.
"We've always had a healthy number of foreign managers in the Premier League and I've always felt we should have a similar balance of British managers working abroad. It may help and enhance me to go abroad... I'll be taking my time finding something that's right,” he said.

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