The Canadian-born Englishman was expected to retire once his contract expired at Old Trafford in June but signed for the Red Devils' city rivals shortly before the transfer window closed.
Constantly dogged with injuries that stemmed from a recurring patellar tendinitis problem since signing for United back in 2007, Hargreaves has been plagued with various setbacks and managed to appear just twice since 2010.
Notably concerned about his ability to continue playing, the 30-year-old followed his injury-free debut for Roberto Mancini's side by telling reporters about his lengthy ordeal.
"It has been three years and four months," he said. "That is frightening in itself.
"I am not surprised how quickly I was in contention. There was a bit of a misconception and I don't think people would believe me if I said everything that went on.
"I would never have anticipated all these setbacks in my worst nightmare.
"You are left to pick up the pieces. That has been the biggest obstacle. It was a pretty humbling experience but I knew this day would come. Hopefully I will learn from it and make some changes and look forward to the future."
Over the summer, Hargreaves’ status as a free agent prompted him to post a video demonstrating his fitness on YouTube, an approach that eventually culminated in both Roy Hodgson at West Brom and Roberto Mancini lodging enquires before the midfielder moved to Eastlands.
"I just needed to be pointed in the right direction," he said.
"The staff have done a terrific job of making sure everything was sorted. I did a lot of hard work this summer as well.
"I just need a bit more time to get used to the tempo. It is more dynamic when you are training with the group rather than on your own.
"So far it has been a great progression."
