The Brentford boss has, for once, conceded that the game is more important that winning a league match.
"Everybody says the league is the bread and butter but this FA Cup game is the equivalent of a ship load of Mothers Pride and a pallet load of margarine," Allen said.
The game is being screened live on Sky, which should earn Brentford £75,000.
They face a Bristol City side who have put together a 12-match unbeaten run. Allen said: "They are the Chelsea of League One."
Deon Burton is suspended and Steve Hunt, Michael Dobson, Eddie Hutchinson and Scott Fitzgerald are all ruled out through injury.
