Fellaini has been in the best form of his Man United career recently, starting every game since appearing as a half-time substitute against West Brom in October.
Just minutes into his return, Fellaini scored his first goal for the club and has since started matches against the likes of Manchester City, Chelsea, Arsenal and Liverpool.
Positive performances have got a large majority of supporters back on side after an awful first season at Old Trafford but the midfielder could end his run of games this weekend.
“Fellaini is ill,” told MUTV after his press conference.
Van Gaal will want as many of his squad available as possible given United have four games in ten days after Christmas, three of them away from home in quick succession.
The club still have injury issues but van Gaal did confirm to MUTV this morning that 22 of his squad are fit and available for the Aston Villa match.