Sunderland goalkeeper Jordan Pickford has been ruled out for up to eight weeks with a cruciate ligament strain.
Pickford hurt his knee in a collision with Papy Djilobodji as Zlatan Ibrahimovic scored Manchester United's second goal in a 3-1 Premier League defeat on Boxing Day.
The 22-year-old completed the remaining eight minutes of the match but now faces a period on the sidelines.
"Jordan Pickford has good news and bad news," Moyes told a news conference.
"The good news is he hasn't ruptured his ACL, the bad news is he's strained it.
"We're looking at six to eight weeks, it's a major disappointment for him and for us because he's been saving us points."
Vito Mannone will fill the void left by Pickford and Moyes has confidence in the Italian.
"Vito is fine and he's been training for many weeks since his injury. He has a lot of experience," he added.
Providing fitness updates on the rest of his squad, Moyes commented: "Jack Rodwell is back in training and Javier Manquillo and Steven Pienaar have also been involved."
Sunderland, who sit two points adrift of safety, visit Burnley in the Premier League on Saturday.
‘After Manchester City’s recent form, maybe they’re the underdogs against Manchester United!’ Former Red Devils defender on this weekend’s derby
‘Arteta, Alonso, Emery, me… none of us were physical players – we needed the understanding of the game. That probably helped us move into management’: Premier League boss reveals reasons for natural career progression
