The Spain keeper, who had conceded a soft goal in last week’s 1-1 draw with Everton, allowed the weakest of shots from Ismaila Sarr through his hands in the 50th minute.
A terrible blunder from Manchester United goalkeeper David de Gea helped Premier League basement side Watford to a 2-0 victory on Sunday for their first home win of the season and maiden victory under coach Nigel Pearson.
The Spain keeper, who had conceded a soft goal in last week's 1-1 draw with Everton, allowed the weakest of shots from Ismaila Sarr through his hands in the 50th minute.
Senegalese winger Sarr then earned the penalty from which Troy Deeney doubled Watford's lead in the 54th minute, sending De Gea the wrong way and blasting into the net.
United, who have failed to build on the momentum from their back-to-back wins over Tottenham Hotspur and champions Manchester City earlier this month, produced yet another limp display and failed to hit the target in the first half.
Club record signing Paul Pogba made his first appearance since facing Arsenal on Sept. 30 by coming on in the second half but failed to lift his side, who barely managed to trouble their former goalkeeper Ben Foster in the Watford goal.
United's fifth Premier League defeat of the season leaves them eighth in the standings on 25 points after 18 games, while Watford are still bottom but level on 12 points with 19th-placed Norwich City.