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Quique Sanchez Flores' Watford were too often narrow and stretched, even though the Spaniard oversaw an improvement in the second half.
It was not his fault Ighalo's finishing disappointed.
Watford attempted to hit back but were limited to Deeney's curler over the crossbar and Allan Nyom's deep cross, which caused momentary concern in the Palace area before Joel Ward turned the ball behind.
The Hornets suffered a major blow after half an hour when midfielder Etienne Capoue, an influential figure all season, collapsed in agony after a seemingly innocuous off-the-ball collision with Bolasie.
They should be offering him a new contract and encouraging him to build."We've earned that right. They probably weren't that the last time, in 1990, but they are now so it's a big challenge for us and the real challenge for us today was to get to the final.