Goalkeeper Ben Foster admitted Ashley Young would be 'a massive loss' to Watford if, as expected, the forward leaves the relegation-threatened club this month.
The Hornets had accepted a bid from West Ham worth up to £9.65million for Young, but the 21-year-old is thought to have turned down the chance to move to Upton Park.
Aston Villa are sure to have been alerted, while Spurs - whose interest has cooled recently - could also rejoin the race.
But goalkeeper Foster knows the departure of Young would deal a huge blow to Watford's hopes of surviving in the Barclays Premiership.
'Ashley is a really important player for us, and a huge talent,' said Foster, who is on a season-long loan from Manchester United.
'I am sure he can go on to bigger and better things in the future. But, while he is here, we really do need him.
'He is a vital part of our team. A lot of the play goes through Ashley, and he would be a massive loss for us if he were to leave.'
Foster, who has become a feature in the England squad this season, has been one of the few players to shine for Watford in a difficult season at Vicarage Road.
Following the 3-0 home defeat by Liverpool at the weekend, the 23-year-old has begun to wonder whether his team-mates still possess the stomach for a fight.
He continued: 'A few people need to have a good, hard look at themselves, and make sure they are up for the fight we have ahead of us.
'We have been trying to work as hard as we can all season, trying to grind teams down. That is the key to our play.
'Against Liverpool, I don't think there were many players who could say they gave 100%.
'We need to have a good talk about things, to try to work out what went wrong in that match.'