Following defeats by Swansea and Barcelona, this was Arsenal’s third consecutive loss at the Emirates – their worst home run since moving from Highbury in 2006.
It was also their first FA Cup defeat since February 2013 when they lost to Blackburn – 756 days and 16 matches ago.
Is Arsenal’s season over?
Defeat here to Swansea in early March saw Arsenal’s Premier League aspirations take a serious blow, and with the gap to leaders Leicester having since moved out to eight points, winning the title looks very unlikely.
Then followed their 2-0 reverse at home to Barcelona in the last 16 of the Champions League. Not many Gunners fans will be betting against the Spanish side progressing at their expense in the second leg on Wednesday.
Victory in this year’s FA Cup was without a doubt the club’s best chance of silverware. And it would have been Wenger’s seventh triumph, making him the most successful manager in the history of the competition.
Now, though, the pressure will be on the Frenchman like never before during his near 20-year reign in north London.
If Arsenal’s two consecutive FA Cup trophies were supposed to demonstrate progress towards winning the league or the Champions League, what does this season’s disappointing exit herald?
How a familiar goal changed the game
In the opening period Ighalo – who had not scored since January during the fourth-round win against Nottingham Forest – looked a pale shadow of the player he was in the first half of the season.
How goals transform a striker’s confidence though, and when the opener came it had a very familiar feel about it.
Strike-partner Troy Deeney provided the assist, as he has for so many of Ighalo’s 17 goals, rising above both Calum Chambers and Per Mertesacker to flick on to the Nigerian, who spun away from Gabriel to fire home on the turn.
It was a familiar goal for Arsenal, too. Once again they had gone behind after failing to make their early dominance tell, and once again the defending was soft.
No such criticisms could be made of Watford’s second though; Guedioura’s stunning strike soared into the top corner.
But clearly Arsenal were left vulnerable by the chances they failed to take, with Mohamed Elneny spurning two clear first-half openings.
Man of the match – Odion Ighalo (Watford)
What the managers said
Arsenal manager Arsene Wenger: “Our fantastic run has come to an end on a day where we felt that our performance deserved a win. Credit to Watford, they created few chances but took them all. It’s sad because we gave everything.
“The players don’t miss on purpose. You have to be realistic. Of course on the first goal, we made a mistake.
“It is very difficult against Barcelona, but we have to see it as a challenge and try to make the impossible possible.”
Watford manager Quique Flores: “It was difficult in the first half, we tried to contain Arsenal and then try to create attempts and play well.
“In the last two months we were being competitive, but not winning. I feel fantastic, I’m really happy for the players and the fans. They follow us and it is an amazing prize for them.
“I have memories on television of the old Wembley and watching in Spain and seeing the finals of this competition. I’ve just arrived in England, and to get there is exciting.”
The stats you need to know
Watford have reached the FA Cup semi-final for the first time since 2007, where they lost to Manchester United.
Arsenal have lost three successive home games in all competitions for the first time since November 2002.
The Hornets earned their first win against Arsenal since April 1988.
Troy Deeney has eight assists in all competitions this season, five more than any other Watford player.
Everton’s Ross Barkley (10) is the only English player in the Premier League with more assists in all competitions than Deeney this season (eight).
Arsenal’s next match is Wednesday’s Champions League last-16 second leg away to Barcelona (19:45 GMT), with the Gunners 2-0 down from the home tie. Watford next play at home to Stoke in the Premier League on Saturday (15:00 GMT).