The youngster has staked his claim for a regular first team spot this season.
He was vital for City in their 1-0 win over Olympiacos in the Champions League away from home.
Foden got the only goal in the first half after latching onto a clever Raheem Sterling back heel.
"I think Phil Foden is gonna become undroppable," former City man Hargreaves said on BT Sport.
"I would love to see Bernardo Silva get that form back that he had a couple of years ago, when he was one of the best players in the Premier League.
"I think they're close, and those chances they're creating, they're gonna get more clear-cut and we're gonna get more cricket scores in the future."