Romelu Lukaku continued a fine start to his season by netting three times for Belgium and Marcus Rashford scored twice for England, against Spain and Switzerland.
Rashford has struggled for time on the pitch at United this season with Mourinho able to pick from a clutch of attacking options including Lukaku, Alexis Sanchez, Anthony Martial and Juan Mata.
But the United boss will be spared the difficult decision of whether to play Rashford over the next few weeks, with the 20-year-old starting a three-game ban against Watford following a red card in the Burnley match.
Mourinho could be also be without defender Luke Shaw, who suffered a nasty head injury in England's match against Spain on his return to the national side.
Watford brushed aside Brighton, Burnley and Crystal Palace as they made a perfect start to the season.
But their real statement of intent came in their come-from-behind win against Tottenham that enabled them to keep pace with Liverpool and Chelsea.
Watford boss Javi Gracia, though, warned his side of under-estimating United.
"Nowadays we talk about the coaches a lot more, but they have a great squad, many internationals, recently crowned champions of the world," said the Spaniard.
"We've seen them in all the big international games and we know it is going to be a very difficult game. But we will try to compete in the same way we have done things in the last couple of games.
"I think that as soon as we begin to think the next game is going to be easier for some reason, we are dead... we will pay heavily for any sense of over-confidence against a team like United."