Saturday's disappointing defeat at already-relegated Portsmouth confirmed United will take part in the end-of-season play-offs.
The loss and results elsewhere made it mathematically impossible to catch the division's top two.
Three goals in 11 first-half minutes, from Shaun Cooper, David Connolly and Jed Wallace, stunned the Blades and the 1,836 Unitedites inside Fratton Park.
With Maguire banned due to the two yellow cards picked up in midweek, Higginbotham took over in central defence - the only change to temporary boss Morgan's starting XI, with Westlake was promoted to the bench. Pompey, meanwhile, were skippered by former Blades utility player, Ertl.
Buoyed by a bumper attendance, Portsmouth made an enterprising start, winning an early corner and having a penalty appeal turned away when McDonald challenged Sodje inside the opening five minutes.
United appeared to have weathered the early storm and went close when Porter met a corner from McMahon but two goals in the space of three minutes put the home side firmly in the driving seat.
Firstly, after Wallace had held off McDonald, COOPER rose above McMahon to head home Agyemang's right-wing cross and then Wallace was the provider of the neat-post cross for CONNOLLY to nip in ahead of his marker to double the advantage.
In the next three minutes Connolly fired into the side-netting and Wallace shot straight at Long after Ertl had beaten McDonald but Pompey did add a third just after the half hour mark. Two defenders failed to cut out Connolly's low cross from the left and WALLACE found himself in ample space to slide the ball home past the exposed Long.
Half-chances fell to Porter and Higginbotham, the former having a shot deflected wide and the latter's header failing to find the target from the subsequent corner, whilst on 42 minutes, the final chance of the half, McMahon picked out Porter but the striker was called offside as he headed into the goal.
At the start of the second half the Blades made a double change, Forte and Robson taking the places of Flynn and Kitson, and chasing the game the visitors attempted to put Pompey under pressure. Sodje headed over his own crossbar to prevent Porter connecting with Murphy's cross and McMahon's half-volley failed to trouble goalkeeper Eastwood.
It was McMahon's last contribution as he limped off soon after, Westlake taking over at right-back, and Pompey attempted to reassert themselves, with Wallace going close on two separate occasions.
McDonald was the first Blade to pick up a booking, the referee signalling an accumulation of offences, whilst Morgan switched to a 4-3-3 formation with Forte and Murphy joining Porter in attack. On 70 minutes Higginbotham headed over a Robson corner but the Blades were unable to seriously test Eastwood.
Late on it was the Pompey who were seeing more of the ball but the only chance of note saw Long punch clear a Wallace corner. Three minutes of injury-time saw the Blades win a couple of free-kicks but again the home side were barely troubled.
Pompey: Eastwood, Cooper, Sodje, Connolly, Moutaouakil, Gyepes, Ertl (Awford 90), Racon, Agyemang (Harris 86), Butler, Wallace (Maloney 89).
Unused: Smith, Walker, Akinde, Webster.
Blades: Long, Higginbotham, Hill, Flynn (Forte 45), Doyle, Porter, McDonald, Kitson (Robson 45), Collins, Murphy, McMahon (Westlake 59).
Unused: Ironside, McFadzean, Whitehouse, Coyne.