Moyes, in an exclusive with the Daily Mail, said he moved from Everton to Manchester as he believed he had time to build a successful team, after predecessor Alex Ferguson stood aside after 27 years in the job.
But the Scotsman labelled the job "impossible", as United let him go less than a year into his six-year contract in April.
Moyes was speaking in the days leading into Saturday's Premier League curtain-raiser, which saw Swansea City trump United 2-1 at Old Trafford.
"It was a step into the unknown and, looking back now, it was near enough the impossible job," Moyes said.
"But it was the right job for me. I'd been at Everton for more than 11 years. We'd qualified for the Champions League, got to an FA Cup final, I'd been voted manager of the season three times. I was among the most experienced managers in the Premier League. United had always had British managers.
"I was devastated to lose the job because it was something I felt I 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. In the end, I don't feel I was given time to succeed or fail."
Moyes said he saw similarities at United with long-time club Everton, where he conceded his start was also rusty before he lasted 11 years.
"It was not easy to begin with at Everton either. We had a couple of tough years before we had the club the way we wanted it," Moyes said.
"But the reason I went to United was because I thought I was joining a club that would give me time. That was the big thing.
"I didn't want to change anything immediately. I wanted to take my time working out what I thought was needed."
Moyes said he would continue to follow the club's fortunes after his first club-less close-season since he was 16.
"It's going to hurt this weekend that I'm not managing Manchester United, because that's obviously what I'd have liked to have been doing," he said.
"But I'll watch the games. I won't be running away from it. Football is something I've been involved in since I was a boy."
