Northern Ireland international Grigg stunned Pep Guardiola's team, scoring the winner to inflict just a third defeat on the club this season.
Grigg has hailed his team's "unbelievable" performance, adding that the Latics stopped one of the best teams in Europe.
"It's got to be the biggest upset in a long time," the 26-year-old told The Mirror. "I think it's bigger because we're a League One team and they are probably the best team in Europe at the moment.
"They are flying, the quality of players they've got, they are an unbelievable side, scoring goals for fun. So for us to beat them on our patch is something special.
"It was an unbelievable performance — everyone to a man, including the substitutes. The gaffer and his staff worked really hard all week on a game plan. The promotions are up there, but in terms of a single game and a single moment, this is probably it."
City, who are still on course to win three trophies this season, have the chance to win their first piece of silverware in the EFL Cup final on Sunday.