The Hornets know a victory at the weekend will allow them to move level on points with Burnley, but they face a difficult trip to Turf Moor.
Watford head into the game with confidence after holding Tottenham to a 1-1 draw last week, but Mariappa concedes Burnley will pose a totally different challenge.
Mariappa, who used to play under Clarets boss Sean Dyche at Vicarage Road, has urged his team to prepare for a fight ahead of kick-off.
He told the Watford Observer: “I don’t think our battling nature has ever been in question this season and we have had to battle at other places this season.
“We have shown great character and we know it will be an extremely tough game, but another challenge we are looking forward to.”
Sheffield United could have leading scorer Leon Clarke back for their home game against Nottingham Forest.