Lukaku's point-blank finish with his midriff opened the scoring in the 35th minute and three minutes later Smalling had the Red Devils roaring.
"Credit to Watford", said Lukaku.
When asked if he believes Rashford has earned more playing time after scoring twice for England over the worldwide break, Mourinho gave a sarcastic response, making his feelings towards the media well known.
"They are both doing well, they are both different players and I think they both understand that one is a young player and the other is the experienced one and it is good for us to have these two options, plus [Matteo] Darmian".
The result leaves Watford fourth, while United climb to eighth.
In contrast, Smalling showed the technique and poise of a seasoned striker to bring down Fellaini's knock-down on his chest and volley into the roof of the net.
Mourinho was full of praise for Fellaini after yesterday's match.More news: Strawberry crisis spreads to NSW as boy finds pin in Coles punnet
"But for the Manchester United supporters it is important they have the right idea of how things are in reality but the most important thing of all is that the kid is a good kid, a good player, knows what Manchester United did for him".
Premier League football returns after the worldwide break as Manchester United take on high-flying Watford at Vicarage Road this evening.
Two of the players Mourinho mentioned in his defence of Rashford, Solanke and Loftus-Cheek, had previously failed to break into the first team at Chelsea during the Portuguese's second stewardship at Stamford Bridge.
Jose Mourinho has kept nearly the entire starting eleven from Manchester United's win over Burnley.
Javi Gracia names an unchanged Watford team to the one that beat Spurs before the global break.
United were in the ascendency ten minutes into the second stanza but they were yet to create clear chances in that period.