The midfield enforcer had powered home a second-half equaliser for the hosts to cancel out Sheffield United U21s’ 10thminute opener courtesy of Daniel Philliskirk.
However Terry Kennedy bundled the visitors’ winner over the line in the 68thminute after Eric Tonne’s corner had triggered a melee in Blues’ six-yard box.
Richard Beale had fielded an experienced starting XI with Diop, Darren Ambrose and Nikola Zigic, who all played 69 minutes, filling the three overage outfield berths, while Colin Doyle took his place between the sticks.
Reflecting on the defeat, Beale told bcfc.com: “We’ve conceded two very poor goals as a result of individual errors and you don’t win games of football if you do that.
“It’s disappointing but it’s not about the result. The good thing is that it has given Ziggy his first 70 minutes since his suspension, Papa and Darren Ambrose also needed that time as did one or two of the younger lads who have been around the first-team squad.
“They all put a good shift in and tried as hard as they could. I wouldn’t knock anybody’s application today, we were just a little bit short of quality and that’s what ultimately wins matches.”
The Blades were bright throughout and Tonne took advantage of a misplaced Diop pass to set up Philliskirk for a smartly taken goal from 18 yards.
However, after that, Blues rallied and had the bulk of the chances in the first-half. Ambrose, who started the game on the left flank, tried his luck from distance on three occasions while Zigic couldn’t keep his volley sailing over after Marcel Henry-Francis’ downward header.
Henry-Francis and Zigic linked up well again just before the interval with the giant Serbian only being denied by a late block from Kennedy.
Blues did manage to find a way through four minutes after the restart. Hancox’s perfectly delivered ball from the left was met by the head of Diop at the far post giving Blades keeper George Willis no chance.
Lively Blues striker Henry-Francis was brought down by Willis two minutes later but despite loud appeals to the contrary referee Alan Sarginson waved play on.
Both sides continued to keep those in attendance at Wast Hills entertained with end-to-end action but it was Sheffield United who were the ones celebrating in the 68thminute..
Kennedy’s initial effort, as a result of Tonne’s corner from the right not being claimed by Doyle, was blocked on the line by Jack Deaman but the Blades’ man followed it up by forcing home the rebound.
Further chances came for both sides; Henry-Francis sent a dangerous low shot inches wide before Tonne’s well-struck free-kick was awkwardly dealt with by Doyle yet Beale’s side slipped to only their second defeat of the season.
Blues U21s: Colin Doyle; Jack Deaman, Emmitt Delfouneso, Will Packwood, Ben Gordon; Callum Reilly, Papa Bouba Diop (Josh Hawker 69), Mitch Hancox, Darren Ambrose (Ryan Higgins 69); Nikola Zigic (Reece Hales 69), Marcel Henry-Francis.
Subs not used: Brice Ntambwe, Liam Truslove.
Goal: Diop (49)
Sheffield United U21s: George Willis, Jordan Hodder, Tyler Williams (Aaron Barry 70), Matthew Harriott, Joshua Williams, Terry Kennedy, Eric Tonne, Max Pemberton, Joe Ironside, Daniel Philliskirk, Diego De-Girolamo.
Subs not used: Ethan Lynch, Jahmal Smith, Otis Kahn, Connor Dimaio.
Goals: Philliskirk (10) and Kennedy (68).