Season ticket holders had until 5.30pm today to secure their seats, with any which have not been taken up by the deadline able to be re-sold.
The quantity of seats is believed to be in the hundreds, with limited tickets in the South Stand and East Stand, plus a block in the North Stand which is showing as available.
They are available online tonight, with any remaining tickets on sale in person or by phone from 9.30am on Tuesday.
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Blackburn reported earlier they had 500 tickets remaining from their allocation of 1,500.
It means while the attendance may not quite reach the sell-out of 11,673 from the previous round against Swansea City this month, another five-figure crowd is expected.
United have not reached the last 32 since 1999.
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