Rotherham will be looking to put right a 2-1 defeat from earlier in the season when they battered the visitors in a controversial encounter, only to lose out to two Deon Burton goals.
Allen was guilty of winding up the Millers fans and players that day with a few choice words, but he acknowledged the potential threat ahead of tomorrow's clash.
"I saw them play at Oxford the other week and they were 4-0 up at half time so they are dangerous. They have good players and we have got to be right on it from the first moment", he told gillinghamfc.com.
“We are taking on a big team, a good team. Everyone from the players, the staff and the supporters will have to be on our game on and off the field because they are dangerous, and we will be ready."