United went 3-0 down before the break, with Colin Daniel notching twice and Alfie Beestin adding another for the National League hosts.
And although United staged a late fightback with goals from substitutes Rhian Brewster and Iliman Ndiaye, Paul Heckingbottom called the defeat a reminder of the standards that he expects from his players when the season starts for real next month.
United return to action on Tuesday evening against Mansfield. Here’s how we rated United’s players in the defeat to the Iron …
1. SCUNTHORPE, ENGLAND - JANUARY 06: A general view of the stadium exterior prior to kickoff during the FA Cup Third Round match between Scunthorpe United and Chesterfield FC at Glanford Park on January 6, 2015 in Scunthorpe, England. (Photo by Clive Mason/Getty Images)
Sheffield United lost 3-2 at Scunthorpe United this afternoon (Clive Mason/Getty Images) Photo: Clive Mason
2. Jordan Amissah 6
Taking his place in goal with both Wes Foderingham and Adam Davies injured and had to be alert to come out and look to boot clear a ball over the top, which hit a Scunthorpe striker and rebounded dangerously before Robinson headed clear. Had no chance with any of the home side's goals Photo: Simon Bellis
3. George Broadbent 4
Was culpable for Iron's opener as he missed the cross and saw Daniels capitalise by rifling home. He didn't cover himself in much glory for another goal either and won't look back at this afternoon with much fondness, although in mitigation he is playing out of position Photo: Simon Bellis
4. Chris Basham 5
Fought off the challenge from Anel Ahmedhodžić to start at right centre-half, but couldn't see it through to half time as he succumbed to injury and was replaced by the new boy before the whistle for the break. United will be crossing their fingers that it's nothing serious Photo: Sheffield United