A bid belived to be £1.5 million for the Hornets striker has been rejected by the Championship side.
"If I were Darius I could understand the fact he would want to play in the Premiership," Hornets boss Aidy Boothroyd told his club's official website.
"The place for him to contribute and to do that is at Watford.
"There are two years left to run on his contract. He has been offered an improved deal with an extra year."
"We've made a good start and he's been important in that start and as far as I am concerned it's time to be professional and get on with it."