The Dutch international picked up the injury during the club's first game of the season against Swansea City, and has since been unable to feature for City during their unbeaten start to the season.
But Mancini says that despite being ruled out of the visit of Everton, De Jong has every chance of featuring against Bayern Munich in the Champions League next week.
"I don't know what's happened with Nigel," Mancini said. "In the last week he improved a lot, I hope (he returns) because Nigel is important for us.
"He had the pain for four weeks for this, he started two or three times to train but overtime he had a problem, he had pain and it was difficult for him to run, but now he is ok."
When the Italian was asked if the tough tackling midfielder would be available this week he said: "No, not tomorrow, but maybe the match in Munich or Blackburn.
"Nigel is different from the other midfielders, because he work hard in the middle, and cover a lot of balls. We can attacks with more players because we have Nigel there to help the defenders."
Despite winning four of their first five Barclays Premier League match, and looking fantastic in doing so, Mancini insists he won't understand the true potential of his side until later in the season.
"It's difficult, I think that you need to see game after game because every game is difficult and every game is important, I think we need another two months."