United have already lost twice this season and face the red hot Hornets, with four wins from four, at Vicarage Road in their fifth game of the season.
Fellaini was expected to leave last summer but ended up signing a new deal to extend his stay in Manchester.
And the Belgium ace insists United can win the title despite already slipping behind the leaders.
"We have the quality, that's for sure," said Fellaini. "The season is long. It's the beginning of the season.
Man Utd news: Marouane Fellaini has withdrawn from the Belgium squad with injury (Image: GETTY)
Man Utd news: Marouane Fellaini impressed in the win over Burnley (Image: GETTY)
“The last two years we started very well. This season, a bit less, but the season is long so let's keep going.
"The next game is more important than this one so let's go with the national teams and come back very strong.
“In the Premier League everything is possible so let's see. Like I said, the next game is more important than this one."
Pressure had mounted on Mourinho following back-to-back defeats to Brighton and Tottenham but it is understood his future is not currently on the line.
United are down in 10th place after four games and just two victories - against Leicester and Burnley - and will fall further off the pace unless they beat Watford.
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