United maintained the unbeaten start to the Championship season thanks to an untidy late goal at Derby on Saturday.
The goal was initially credited to Matt Kilgallon but replays suggested the ball came off Rob Hulse's head before looping over Stephen Bywater.
United face - and lost to - Derby on the same weekend last year, after coming back from September's international break.
Harper made his debut in the Saturday lunchtime kick-off at the expense of Evans, who dropped to the bench. There was no place for either Stewart of Hendrie, who swapped clubs on transfer deadline day. Former Blade Hulse started in attack for Derby.
The inclusion of Harper gave boss Blackwell options in his formation and it was quickly apparent that a 4-5-1 system was preferred with Treacy and Ward supporting Henderson from the flanks.
A chance for both sides in the opening two minutes gave encouraging signs. For Derby, Teale was allowed to run but saw his shot deflected for a corner, whilst Bunn's long clearance was headed down perfectly by Henderson but Treacy's shot was sliced. The next two chances also fell to the loan winger but on both occasions he missed the target with right-foot efforts.
Tireless midfield workhorses Montgomery and Savage were booked for fouls on Pearson and Quinn respectively, whilst United thought they had snatched the lead on 19 minutes, only to be denied by a late flag. Ward robbed Addison, breezed through the defence and after his shot was parried by Bywater, Henderson touched into the empty net - only to be deemed offside.
Despite Pearson flicking a difficult volley over from Green's cross, United dominated the opening half hour. An enterprising run from Treacy ended with Henderson clipping just wide at the near post and then the big striker shot into the ground, which was comfortable for the Rams goalkeeper.
Henderson was booked for dissent after a prolonged chat with referee D'Urso, Treacy fired a free-kick well wide, whilst a rare mistake from Walker almost gave Derby a chance to open the scoring just before half-time arrived, following two minutes of added time.
Within 90 seconds of the second half a Teale cross had clipped the top of Bunn's bar but United started well and won a couple of set-pieces, with Quinn firing over from one. The first caution of the second half, on 52 minutes, was picked up by Pearson, who exacted retribution on Montgomery. Morgan was the next in the book, possibly harshly, for kicking the ball away just after the referee had blown for a foul.
Without an abundance of chances, United looked the more dangerous through the mid-point of the second period. A Ward run on 69 minutes gave Henderson a shooting chance but he scuffed his effort and then, moments later, Walker headed over a Treacy corner.
Derby had made two substitutions before Blackwell threw on his first replacement, Evans replacing Henderson as the lone frontman. The home fans thought Hulse had headed in at the back post but the effort was wide and then Teale's long-range shot was held by Bunn.
Just as the game entered the final ten minutes the breakthrough arrived after Barker's clumsy foul on Quinn. Treacy delivered the free-kick into a melee of bodies - including Walker and Kilgallon - and the ball looped up over Bywater's left-hand and into the net. The identity of the scorer wasn't clear immediately, especially as Kilgallon looked embarrassed to claim the goal, and the replays were also inconclusive although the ball appeared to come off Hulse.
It was Treacy's last contribution as he was replaced by Cotterill before the restart and he was soon followed onto the field by France. United, expectedly, slowed down the play and were helped by Commons curling a free-kick into the grateful arms of Bunn.
Four minutes of added time buoyed the home fans, whilst Addison was added to the front line. Cotterill was booked for kicking the ball away but it was incidental as United comfortably held on and could have snatched a second had Ward, who raced clear, got more on a shot at Bywater.
Derby: Bywater, Connolly, Green (Livermore 62), Addison, Croft (Commons 74), Savage, Hulse, Teale, Barker (Leacock 90), Pearson, Moxey.
Unused: Deeney, McEveley, Buxton, Dickov.
Blades: Bunn, Montgomery, Morgan, Kilgallon, Henderson (Evans 75), Harper, Taylor (Cotterill 80), Ward, Treacy, Quinn (France 88), Walker.
Unused: Reid, Bennett, Little.
