Fellaini was a surprise omission from the United squad as they travelled to Vicarage Road looking to close the gap on league leaders Manchester City on Tuesday (28 November). Jose Mourinho's side survived without the Belgium international, cruising to a three goal lead inside the first 45 minutes before surviving a late scare to seal a 4-2 win but the United boss had earlier set alarm bells ringing with his pre-match comments where he said he did not know if the midfielder would be available for Saturday's [2 December] showdown with Arsenal.
According to Het Laatste Nieuws, it was another knee problem that kept Fellaini out of action on Tuesday, with the extent of the problem not yet known by the club.
Fellaini missed six games in all competitions with a knee injury earlier this season. The 30-year-old suffered a medial ligament injury during October's international break and was forced off early on in a 2018 World Cup qualifying match against Bosnia and Herzegovina, missing the rest of the month as a result.
He returned from that knock just before November's international break, coming off the bench in the 1-0 defeat to Chelsea on 5 November. The Belgian went onto start in the Champions League defeat to Basel on 22 November, making further appearances off the bench against Newcastle United and Brighton either side of that defeat in Switzerland.
Mourinho will likely provide a further update on the former Everton midfielder when he addresses the press at his pre-match press conference on Friday [1 December].
United are also sweating on the fitness of another midfielder in Nemanja Matic, who limped off in the victory over Watford with a muscular problem. While the club are still to provide an update on the Serbia international, Mourinho's initial reaction did not bode well for him featuring against the Gunners.
"For Matic to tell me he wants to be replaced, he does not do that if it is nothing important. It is muscular," Mourinho said.