In truth, the clash between two sides who had produced such sterling performances in the FA Cup third round last weekend was a disappointing affair.
But Knight, a former Sheffield Wednesday player, returned to Yorkshire to haunt the Blades with his fourth of the season deep into injury-time.
Richard Carpenter had put Albion into the lead on the stroke of half-time, providing a classy finish to a superb break from a United corner.
But Neil Warnock's side are made of stern stuff and they thought they had salvaged a point thanks to Kerry Mayo's unfortunate own goal just before the hour.
But it was not to be and Knight's late strike put them further behind the pack chasing for the dream ticket to the Premiership.
Warnock made only one alteration to the side which beat Aston Villa last Saturday, captain and centre-back Chris Morgan coming in at the expense of Derek Geary.
And it was home full-back Leigh Bromby who had the first chance after just five minutes.
A surging run by Michael Tonge saw last week's two-goal hero Andy Liddell mis-hit a cross into the path of Bromby, who could only slam his effort wide from six yards.
It took more than 10 minutes for the visitors, unchanged following their 2-1 defeat to Tottenham Hotspur last weekend, to have their first shot on goal and even then Adam Hinshelwood's ambitious 30-yard strike was easily snapped up by goalkeeper Paddy Kenny.
Albion captain Charlie Oatway then had a fierce header diverted away from goal by team-mate Gary Hart while United, looking dangerous on the break, saw Tonge fire into the side-netting from 20 yards.
It came as no surprise when Brighton took the lead on the stroke of half-time. After a United corner was well-cleared, the visitors found themselves three-on-one in the Blades' half.
Knight cut in from the left and fed Carpenter who found the corner of Kenny's net with aplomb.
United started the second half brighter and Tonge's run ended with a shot which just drifted wide of Michel Kuipers' right-hand upright.
Warnock was forced into a change after 50 minutes when Alan Wright felt the full force of a sturdy Carpenter challenge and was carried off on a stretcher and replaced by former Everton striker Danny Cadamarteri.
A comedy of errors led to the visitors surrendering their lead with a goal of their own making in the 58th minute.
Adam Virgo's clearance was hooked back into his own area but as Mayo tried to clear he succeeded only in poking the ball over stranded keeper Kuipers and into the back of his own net.
Albion almost went behind when Liddell's inviting centre found Tonge, whose -header crept agonisingly wide of the far post.
Andy Gray nodded over an Alan Quinn cross when he should really have done better while, in the dying stages, but Kuipers got down to smother another effort from the player.
Knight eventually made the Blades pay for their missed chances. He got on the end of a superb slide-rule pass by Paul Reid deep into stoppage-time to guide the ball past Kenny and give Brighton a much-needed win.