Billy Davies had come under fire for the first time as the Reds suffered their third straight away defeat at Doncaster on Saturday.
But an early strike from Rob Earnshaw was enough to secure three points from fierce rivals United - who played most of the second half with ten men after seeing Darius Henderson sent off for an elbow in the 46th minute.
Forest took only four minutes to surge into the lead, as they began with great urgency.
The goal came as two players restored to the side by Billy Davies combined after Raddy Majewski had delivered a corner towards the near post, with Paul Anderson heading down for the predatory Earnshaw to fire home from close range.
And it was entirely one-way traffic throughout a first half in which the blunt Blades failed to muster a single effort on goal.
Forest will have wondered how they had not taken more of an advantage, after repeatedly carving out openings, largely through the ingenuity of Pole Majewski, whose delivery from set-pieces had United stretched.
Anderson flashed a shot narrowly over the bar following Forest's most incisive move, which saw Chris Gunter turn sharply on the right before picking out Chris Cohen, who in turn squared across goal for the winger to strike.
Cohen himself had also come close with an early effort, bending a left-foot shot just a few feet wide of the post from the edge of the box.
Lee Camp had been forced to make one save, as a Nyron Nosworthy cross eluded his defence and bounced dangerously towards the far post where the keeper palmed acrobatically away.
United's hopes of staging a comeback were dented within 45 seconds of the second half, as Henderson received a red card for swinging an elbow at Anderson.
This sparked a nasty edge to the encounter, as Forest's James Perch and United's Lee Williamson and Mark Yeates were all booked in the space of three minutes.
It took a fine, brave save from Mark Bunn to deny Forest a second after Earnshaw had ghosted through the centre.
Although Camp too was not allowed to let his concentration slip as Yeates let fly with a driven 20-yard strike that the keeper held well, high to his right.
The flurry of goalmouth action continued as Cohen bent in a left-foot shot from the right that had Bunn scrambling across goal to parry, with Anderson snatching at the loose ball and firing it into the sidenetting.
Meanwhile, the Blades keeper produced heroics again to deny substitute Nathan Tyson, palming his rising shot over the bar.