Hargreaves said: "I never would have anticipated [the injuries] in my worst nightmares - it was pretty humbling."
And, after being released by Manchester United, he said it was "essential" for him to "get a change of scenery".
The midfielder crossed the Manchester divide in August after United let him go following just six minutes of first-team football in three years.
And, after an impressive display capped by that fine goal which instantly endeared him to City fans, Hargreaves said: "I feel good.
"It's my first real game action in, I think, three years and four months - which is a bit frightening in itself. [But] I put in a lot of hard work in the summer.
"I've been here three weeks and played two games in six days. We did an in-house game last week, last Thursday.
"They [the medical team at City] have done a great job so far, just easing me in and this is an important progression. Football work is more intensive than individual training [but] whatever these guys think is good going forward.
"It's important to get back into a team situation on a big pitch. Just getting used to basic things you do in the game.
"All in all it was a great exercise for me."
Hargreaves' goal at Etihad Stadium led fans at his new club to chant "Fergie, sign him up", just as they did when Carlos Tevez quit United for City two years ago.
“We need to be calm. After one game after two years he needs some time to recover. But I hope he'll be lucky in the future”
Roberto Mancini on Hargreaves
The goal was Hargreaves' first since April 2008 against Arsenal, and although it was anticipated he would be replaced at the break, the former Bayern Munich and England midfielder lasted for another 12 minutes before departing to a standing ovation.
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