Manchester United central defender Cathcart hurt his knee in the recent home win over QPR and has missed the last two matches, while Cowie has been struggling with illness.
Midfielder Ross Jenkins remains sidelined with the back injury that has forced him to miss the last eight matches, but manager Malky Mackay has no other new injury or suspension problems.
The Hornets, who this week staved off the threat off adminstration by paying back a £4.88million loan to former chairman Jimmy Russo and vice-chairman Vince Russo, are looking to bounce back from two straight defeats in a bid to resurrect their play-off challenge.
In-form Nottingham Forest will take an unchanged squad to Vicarage Road as they look to extend their 14-game unbeaten league run.
Guy Moussi remains the only absentee. The French midfielder has missed the last five matches with an ankle injury that boss Billy Davies this week described as a "mystery".
Davies may opt to make some changes from the home victory over Preston last weekend. However, the Scot will stick with a defence that has kept three consecutive clean sheets in the Championship.
Garath McCleary and Lewis McGugan, who came off the bench during the 3-0 rout of North End, provide options in midfield while Dexter Blackstock, Nathan Tyson and David McGoldrick are available if Davies wishes to freshen up his strikeforce.
Sporting Life