Both clubs were allocated just over 31,000 tickets for the game.
While the Championship club are expected to sell their full complement, Wigan are struggling to sell their section.
A maximum of 6,000 tickets from what was originally a Wigan section will now be sold to neutrals before Saturday's game.
But even with sales in the neutral zone, it is understood the authorities expect a minimum of 14,000 empty seats at Wembley.
Wigan Athletic declined to give details of exact ticket sales when asked.