The Blades have had the highest average home attendance in the second tier this season, narrowly edging out Forest in that respect.
However, The Star understands that, as of Wednesday morning, around 3,000 tickets remain available for the match at Bramall Lane this weekend.
Some supporters may have been leaving it late with there having been long online queues when tickets initially went on sale on Saturday, while there were was also a long line of people outside the ticket office at Bramall Lane earier in the week but that has since subsided as the days have gone on.
It has also been speculated that some fans may have automatically assumed that the match was already a sell-out.
Tickets for the away tie at the City Ground, for which United were allocated 2,000, sold out almost immediately when they went on sale on Monday.
Forest have also sold out their allocation for the tie at the Lane.