Both players missed Tuesday night's 1-0 defeat at QPR and captain DeMerit remains a major doubt with a stomach injury while midfielder Cowie is struggling with a calf problem.
However, the Hornets will be boosted by the return of striker Heidar Helguson, who was ineligible in midweek as he is on loan from Rangers.
Full-back Lloyd Doyley recovered from a hamstring injury to play the second half at Loftus Road and should reclaim his starting spot.
Winger Will Buckley has also been nursing a hamstring problem and could struggle to play again this season.
The Hornets need one win from their remaining two games to guarantee their survival.