Matic was replaced after 54 minutes of Manchester United's 4-2 win at Vicarage Road on Tuesday night, and Mourinho confirmed afterwards the Serbian midfielder had asked to come off.

Ahead of back-to-back Premier League games against Arsenal and Manchester City, the Portuguese coach admitted it is a worry.

"For Matic to tell me he wants to be replaced, he does not do that if it is nothing important," Mourinho told BT Sport. "It is muscular."

Matic has added to a growing injury list at Old Trafford, as Fellaini missed the trip to Watford through injury and is in a fitness race to return in time to face Arsenal at the Emirates on Saturday evening, sources told ESPN FC's Mark Ogden.

Belgium international Fellaini was absent from the travelling party for the trip to Vicarage Road, with United sources unable to specify the nature of the injury. But a source told ESPN FC that the former Everton player was sidelined due to a knee problem, which will be assessed further by United's medical team on Wednesday.

The concern over Fellaini, and Michael Carrick's ongoing absence after undergoing a heart procedure, has left United's midfield options depleted ahead of the Arsenal clash and the visit to Old Trafford of Manchester City on Dec 10.

Phil Jones and Eric Bailly were also missing at Vicarage Road. Jones has missed the last four games after suffering a thigh injury on England duty.

Bailly has been missing since the 1-0 defeat at Chelsea before the international break. The Ivorian defender was expected back against Brighton on Saturday and could return against Arsenal.