The Blades are now six games without a win in League One after falling to a Boxing Day defeat to Port Vale.
A late penalty for Dany N’Guessan handed all three points to the Valiants as United’s promotion push suffered another damaging blow.
They remain inside the top six but sit 11 points off automatic promotion.
The disappointing afternoon got even worse with Blades skipper Chris Basham sent off for a foul on former United midfielder Michael Brown.
A flick from Ben Williamson gave the hosts the lead and it looked set to be a bad afternoon when Basham walked for a foul which United boss Nigel Clough called into serious question.
Jamal Campbell-Ryce equalised from ten yards out to bring some sunshine for the Blades.
But N’Guessan’s penalty, after Bob Harris brought down Mark Marshall, was enough for Vale to snatch the three points.
It was somewhat harsh on the Blades after they walked away with nothing from a battling performance.
Port Vale: Neal, Yates, Dickinson, Duffy, Dodds (Lines 75), Williamson, O’Connor, Brown, Marshall, Veseli, N’Guessan (Daniel 84). Not used: Johnson, Robertson, Birchall, Daniel, Nimely, Campion.
Blades: Howard, Harris, Basham, Doyle, Murphy, Scougall (Reed 70), McGahey (McNulty 85), McEveley, Campbell-Ryce, O’Grady, Cuvelier (McCarthy 46). Not used: Baxter, Higdon,Turner, Adams.
Referee: Stuart Attwell (Warwickshire)