United had, as of the end of last week only sold around 500 tickets for the game.
Ticket prices have been reduced for the game, but tickets must be collected from the ticket office, even for U16s, who's entrance is free.
Entrance for adults is £10 seating and £8 standing with concessions for seniors and U21s charged at £5.
The club may decide to change the ticketing for the clash and make pay on the gate available nearer the evening of the fixture if few more tickets are sold. It is unlikely that Rotherham will be given any more tickets.