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.
