Everton confirmed on Tuesday that the 30-year-old had suffered a foot injury despite playing the full 90 minutes in Sunday's tough 1-0 win against his former club Aston Villa.
Grealish earned the Premier League Player of the Month for August after registering four assists but has since dropped in form, picking up two goals in 17 games.
Moyes spoke on his injury in Friday’s press conference and revealed that is a huge blow to his side who will now have to look elsewhere to replace him.
“It's not for me to give the timescales, we've not got it from the doctors yet. He's got a stress fracture in his foot, that's about as far as I can tell you.
"It's a big blow because he's been hugely important to us since the start of the season. He's been a big part of the team. We've had a few injuries but this is a big blow to us.
Moyes also opened up on whether he'll stay at Everton for his recovery as reports suggest he could head back to City for treatment.
"We're in close contact with Manchester City so we'll see how that works as we go along, and we get more decisions from the specialist."
"We're not thinking about it (replacing him) at the moment, we're only thinking about Jack. We're upset and disappointed for him that he's got an injury."
Everton's injury list still includes Kiernan Dewsbury-Hall, Jarrad Branthwaite, Tim Iroegbunam and Charly Alcaraz whilst Michael Keane is suspended for this weekend’s clash with Leeds United.