Chris Morgan was disappointed with the application and desire of United's U21s following the home defeat to Crewe Alexandra on Monday afternoon.
Mark Howard, on his return to action following a long standing thigh injury, played the first-half before being withdrawn at the interval as a precautionary measure.
The Blades also fielded two trialists, in the shape of midfielder Conor Clifford (Chelsea) and striker Craig Lynch (Sunderland).
A goal in each half saw the Blades slip to a 2-0 reversal at Shirecliffe, with skipper Terry Kennedy giving away a penalty in the build up to the opener.
Speaking afterwards, Morgan says the youngsters let themselves down against the Alex:
"It was a bitterly disappointing performance from the lads and I have told them they need to be better prepared both physically and mentally.
"We lack any real application or desire and without being disrespectful to the opposition we made it an easy win for them rather than Crewe coming out and beating us.
"The tempo we played at allowed Crewe to get a foothold in the game and in truth we were totally off the pace for much of the game.
"It is really frustrating because this performance, compared to the one against Wigan Athletic earlier this month, was like chalk and cheese.
"Hopefully now, with the weather clearing up, we will be able to get some momentum with a good run of games, although we certainly must improve if we are to pick up some positive results.
Blades: Howard (Owen 45), Hodder, McFadzean, Clifford, Kennedy, Barry, De Girolamo (Khan 69), Whitehouse, Ironside, Lynch, Tonne.
Unused: Pemberton, Smith.