But they welcome back Matthew Briggs and Alex Geijo as they bid to finish the season by breaking into the top two places in the npower Championship.
Cassetti was given his marching orders in Saturday's disappointing 3-2 defeat at Peterborough and now serves a one-match ban.
On-loan full-back Briggs was named on the bench at London Road before withdrawing having picked up a groin injury in the pre-match warm-up.
Striker Geijo, meanwhile, missed out altogether due to a knee injury but both are fit for consideration against the Lions.
Otherwise, fit-again midfielder John Eustace is still lacking match fitness after his long-term back injury, while defenders Fitz Hall (hamstring), Nyron Nosworthy (Achilles), Neuton (dislocated shoulder) and Tommie Hoban (ankle) all remain sidelined.
The third-placed Hornets are now six points adrift of Hull with four matches remaining and could see their hopes of a top-two finish disappear completely this week.