Fellaini requires ankle surgery that will rule him out for the rest of the season while Cahill could be sidelined for up to three weeks with a foot injury.
The squad has "one or two other knocks and bruises", according to manager David Moyes, but there are unlikely to be many changes from the group on duty in Tuesday's FA Cup defeat to Reading.
Striker Jermaine Beckford will be back in contention to start after his late arrival for the Reading game limited him to a substitute's role.
Source: PA
Source: PA