The 27-year-old told BBC Radio Manchester he has not reached full fitness since arriving at Old Trafford from Bayern Munich over a year ago.
"I don't think I played a game healthy last season. I was probably at 50 or 60% so there's more to come," he said.
He missed Sunday's 1 - 1 draw against Newcastle because of a knee injury.
Hargreaves also believes he will continue to develop as a player at United.
"I'd like to be fit as soon as possible with the Manchester United games coming up and obviously the England qualifiers are big games for us. I'm working on my fitness every day doing as much as I can, training hard, and hopefully I'll be fit very, very soon." he added
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