The Blades had fought back from 2-0 down to level at 2-2 through goals from Gus Hamer and Will Osula before the break, before Joao Pedro sealed his hat-trick with two more goals in the second period.
Pedro scored twice from the penalty spot, both spot-kicks conceded by Jayden Bogle, before sub Danny Welbeck capped victory with a fifth deep into stoppage time which squirmed through the grasp of Blades debut goalkeeper Ivo Grbic.
United can now at least put full focus on their Premier League survival bid, with Crystal Palace next up for the Blades on Tuesday night. Here's how we rated Chris Wilder's side as they tumbled out of the FA Cup...

9. Will Osula 6.5
Played down the right with Archer more central and looked dangerous again as he looked to build on his two goals in the last round to help United to this stage. Had a couple of openings early on that he didn't take before his run and cross down the right led to Hamer's finish. Nodded home the equaliser from close range before a farcially-long VAR check despite clearly being onside. Will be a real threat for United, especially if he can sharpen up his finishing and require less chances to hit the back of the net | Getty Images

10. Cameron Archer 5
Had his chance to really stake a claim for a Premier League start in his preferred position of down the middle but didn't really take it. He had the odd moment but frustrated teammates and fans alike with his tunnel vision when on the ball, electing to try and go himself when others, including Hamer, were better positioned | Getty Images

11. Ben Brereton Diaz 6
Fresh from his goal last time out, the wide man looked desperate for more but couldn't replicate his West Ham contribution here. Almost picked up an assist with a lovely weighted pass for Bogle but his shot was deflected wide. Looked threatening down the left flank before he was replaced by McAtee | Getty Images

12. Subs: Yasser Larouci 5
Came on for Norrington-Davies on his return from African Cup of Nations duty with Algeria and couldn't do enough to prevent Welbeck bursting past him and netting the Seagulls' fifth late on Photo: Andrew Yates

