Centre-half DeMerit is struggling with a stomach injury and midfield duo Buckley and Jenkins have hamstring problems.
With the Hornets safe from relegation following last Saturday's 3-0 win over Reading, boss Malky Mackay has no need to take any risks with their fitness.
Mackay is considering adding academy duo Piero Mingoia and Gavin Massey to his squad.
Danny Graham and Heidar Helguson scored the goals which secured their survival last week and should continue their partnership in attack. Helguson is due to return to QPR after his loan spell ends but he wants to return on a permanent basis.
Arsenal's Henri Lansbury also plays the final game of his season-long loan.
The match kicks off at 1pm.