The Championship side will have to be at their best to get into the quarter finals.
Watford are flying in the Premier League, currently in 8th place.
But it is possible they will rest a few players, giving QPR a bigger chance in the game.
The former England boss McClaren said to reporters: "Anything can happen in the FA Cup.
"We're at home, it's on a Friday night under the lights, it's on TV. It's an opportunity for certain individuals and the team to show themselves against Premier League opponents.
"Watford are very strong and we'll have to put a top performance in to get any kind of result."