James Wallace completed 90 minutes as United's U21s slipped to a narrow 1-0 defeat against of QPR, who included England international Shaun Wright-Phillips in their side, at Shirecliffe on Monday afternoon.
Stephen McGinn and Chris Porter also played full games in front of the watching Blades management team, but the contest was settled by a goal in the 37th minute, when Junior Morais scored for the R's, managed by former striker Kevin Gallen.
In a quiet first-half, McGinn shot over for the Blades, before the visitors opened the scoring with their first meaningful attack.
MORAIS scrambled the ball home from inside the six yard box after good work outwide. United's response was almost immediate but Chris Porter had his effort chalked off after the referee spotted a foul on his marker in the build-up, meaning the Blades trailed at the break.
The second-half began with the home side on the front foot, Porter curled just wide after controlling a dangerous ball in from the right from Otis Khan.
At the other end Willis had to be alert to make a smart save from the lively Morias, whilst the hosts introduced Daniel Cockerline and Dominic Calvert-Lewin for Jamie McDonagh and CJ Hamilton just after the hour mark.
United continued to press for that elusive equaliser and Khan came closest to rescuing a point with 18 minutes remaining, only to see his left footed strike go just over the bar.
The defeat is United's second single goal loss to Premiership opposition in less than a week, after going down to Crystal Palace at Selhurst Park last Tuesday night, although the Blades will take plenty of positives from this fixture, in particular Wallace playing 90 minutes, his first full game in red and white stripes.
Blades: Willis, Paling, Scarisbrick, McGinn, Berry, Evans, Khan, Wallace, Porter, McDonagh (Calvert-Lewin 64), Hamilton (Cockerline 64)
Unused: Ramsdale, Cheeseman.