Fellaini facing six-month layoff
He sustained the injury after a two-footed challenge from Liverpool's Sotiris Kyrgiakos earned the Greek defender a red card during the Merseyside derby at Anfield on February 6.
Fellaini, distinctive for his huge Afro hairstyle, escaped censure from referee Martin Atkinson after his own studs-up tackle on Kyrgiakos in the same incident.
"He (Fellaini) is not good. He's going to have an operation," Moyes told the club's website ahead of Tuesday's Europa League game against Sporting.
"He's got a similar injury to Robin van Persie and it's probably going to keep him out for six months."
Arsenal's Dutch striker Van Persie has not played since November after rupturing ankle ligaments.