David de Gea once again apologized to supporters following Manchester United's "nightmare" display against Watford and accepts his side's current form is not acceptable.

United fell to a fifth defeat in seven Premier League matches with a 4-1 reverse at the hands of lowly Watford on Saturday, piling further pressure on boss Ole Gunnar Solskjaer.

The defeat at Vicarage Road was United's heaviest against a newly-promoted side since a 5-1 loss to Manchester City in September 1989.

Solskjaer's side have now conceded 29 goals in all competitions this season – only Norwich City and Leicester City (both 30) have shipped more among Premier League clubs.

De Gea, who saved two penalties and was United's best player on the day despite conceding four goals, admits after the latest defeat that a big turnaround is needed.

"There's not much to say – it was embarrassing to see Man United play like we did today," he told Sky Sports. "It's not acceptable – the way we were playing and doing things. 

"The first half was embarrassing – we could've conceded four goals in 45 minutes. It was hard to watch the team playing today – it was nightmare after nightmare. 

"It's not acceptable. We are trying to give our best and fight for the team but for sure something is very wrong. 

"You can see in the games – it's very low level, very poor. Sorry to the fans, once again."

United struggled to cope with high-pressing Watford and were two goals down at half-time through strikes from Joshua King and Ismaila Sarr, who earlier had two penalties saved by De Gea.

Kiko Femenia converted the rebound from the first of those but the spot-kick had to be retaken due to encroachment, with De Gea again equal to the second attempt.

Substitute Donny van de Beek pulled one back five minutes after being brought on, but United then lost Harry Maguire to a second bookable offense with 69 minutes played.

That was Maguire's first sending-off for United on what was his 121st appearance for the club in all competitions, capping what has been a tough month for the center-back at club level.

The Red Devils enjoyed a spell on top without making it count, before Joao Pedro and Emmanuel Dennis sealed an emphatic win for Watford, who started the day in 17th place.

Solskjaer held his hands up towards United's traveling supporters at full-time and appeared to wave, but De Gea insists the players should take the blame.

"It's easy to blame the manager or the staff but sometimes it's the players. We need to show much more than we are doing," he said.

"I've been in difficult moments and always believe in myself but there are difficult moments we have to be strong in mind and body. 

"We are paid to play for United and to do better than that. This is not Manchester United or the way we ought to play – it's difficult to say more than that.

"It's been very bad for a long time – a club like Man United we have to be fighting for trophies and fighting for big things and to be honest we are far from that. 

"Let's see – we have to keep working hard and stick together. We always say the same things but it's the truth – we have to look at ourselves and see where we can improve."