Boss Nigel Clough is optimistic about United's chances against QPR at Loftus Road, but says a win against the Premier League outfit would represent a 'major upset'.
Speaking ahead of Sunday's third round clash, the Gaffer believes the Blades cause won't be helped too much should the home side opt to make changes having played on New Years Day.
He remarked: "Regardless of any changes, the starting eleven Harry Redknapp puts out against us is a Premier League team.
"Even if they make wholesale changes from the Swansea game I don't think it makes our task any easier because then you are facing players who are fresh and with a point to prove.
"If anything, it argubaly makes our life tougher."
On the tie, Clough added: "People seem to be expecting us to go to Loftus Road and win, but we keep stressing to everyone it will be another major upset if we are victorious.
"Of course we have had some exceptional results against Premier League clubs in the last year or so, but it takes an awful lot to beat them, first and foremost we have to be at our best and hope QPR aren't at theirs.
"In cup competitions we don't tend to make the silly, minor errors we keep making in league matches though, that has been the only real difference.
"Hopefully come 3pm on Sunday we will be celebrating with our travelling supporters, but it will take another massive effort from the players.
"QPR are a strong side at home, their record speaks for itself, it is a compact stadium, but that shouldn't faze us, as we are used to that."