Watford were relegated from the Premier League after just one season back after Wilfried Zaha's penalty earned Crystal Palace a 1-0 win on Saturday.
A sixth straight defeat left the Hornets still 12 points adrift of safety with just three matches to play.
Roy Hodgson was welcomed back to Selhurst Park a hero for his four years in charge of Palace, but the former England manager's record of never suffering relegation ended with his final job before retiring at the end of the season.
Patrick Vieira's side climbed up to ninth place on 44 points while Watford remained second-bottom with 22 points and will join Norwich in the Championship, dropping down to the second division for the second time in three seasons.
Palace controlled the contest from the start. Odsonne Edouard nearly scored a superb solo goal just after the hour mark, dancing past the Watford defence before hitting the post from a tight angle after his shot went through Foster's legs.
Watford's task of scoring at least two goals became even more difficult when they were reduced to 10 men after Hassane Kamara, booked for handball earlier, was shown another yellow card for a foul on Michael Olise with around 20 minutes remaining.
"Last week, we were 1-0 up and looked as though we were going to carry that result through but when those last two goals went in... I think only someone who's got no idea what football is about would have believed we could still do it [avoid relegation]," Hodgson said.
"We were, I suppose, resigned to that to some extent last week, and I think that makes today's performance even more commendable... We kept going, we were competitive until the end and that was a good effort from our team."
The 74-year-old Hodgson will leave Watford having failed in his short-term mission to prevent the club making an immediate return to the Championship.
The former England manager was hired in January after the club dropped into the bottom three.
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