The Spaniard, however, was powerless to prevent the Red Devils from being outclassed by their opponents as they raced to a 2-0 half-time lead at Vicarage Road thanks to goals from Joshua King and Ismaila Sarr.
After Man United star Scott McTominay bundled King over in the box with a clumsy challenge after a few minutes on the clock, De Gea smartly blocked his shot before Kiko Femenia followed up to fire in the rebound.
However, VAR ordered the penalty be retaken after ruling that Femenia was in the area when the first kick was taken.
Sarr stepped up to take the second one and De Gea dived low to make a second straight save.
The Hornets eventually got their lead after 25 minutes as King tapped home from close range before Sarr doubled the hosts leads shortly before the break.
While United endured another nightmare, for De Gea it continued the goalkeeper's sudden return to form from the spot after years of penalty agony.
De Gea had remarkably failed to keep out 40 penalties in a run that stretched across FIVE YEARS, until earlier this season.
That dismal record from from the spot included 11 in the shootout defeat to Villarreal in the Europa League last summer.
But his hoodoo was ended against West Ham in September when he saved a crucial and controversial last-minute penalty from Mark Noble, who had been brought off the bench by manager David Moyes only to see his kick saved - with United winning 2-1.
Before that, De Gea's previous save from the spot was in April 2016 against Everton in the FA Cup.
That penalty was taken by Romelu Lukaku, who left Everton to sign for Man United, spent two years at Old Trafford, was sold to Inter Milan, won the Serie A title and then returned to the Premier League with Chelsea, all before De Gea saved his next one.
And now - including his double stop against Sarr - he's saved three penalties in a row.
De Gea has arguably been United's Player of the Season in what has been a disappointing campaign for the Red Devils after their big summer spend.
He has been faultless in most matches this term, but the same can't be said for his teammates, who do not seem to be gelling under Ole Gunnar Solskjaer.
The Norwegian is under huge pressure at Old Trafford with former Real Madrid manager Zinedine Zidane and current Leicester boss Brendan Rodgers topped for the Old Trafford hotseat.
A defeat to Watford could put an end to his reign in the dugout of his former club.