United's U21s slipped to a disappointing home loss against Charlton Athletic at the Look Local Stadium, home of Stocksbridge Park Steels, on Monday afternoon.
A goal in each half from Josh Umerah, either side of a strike on the hour from Ademola Lookman condemned the Blades to defeat at Bracken Moor.
On the eve of kick-off the Blades were dealt a blow when skipper Terry Kennedy withdrew during the warm-up and he was replaced by 16-year-old Callum Semple at the heart of the Blades back four.
The youngster impressed Chris Morgan, who commented: "Callum came in for Tel and was excellent.
"He showed great composure and defended admirably for such a young man. He has got a big future in front of him if he keeps showing the attitude and willingness to learn that he has demonstrated to date."
On his side's performance, Morgan added: "Charlton were the better side on the day, too many of our lads had an off day.
"Saying that, we tried to stay in the game and had a few half chances to equalise, before conceding a second on the break.
"We are disappointed to concede three goals and we can't afford to gift good teams goals like we did.
"At the moment we are striving for that consistency, because we are very up and down. We were excellent at Watford last week and thoroughly deserved our victory, but we didn't play with the same energy or have that cutting edge against Charlton and we got punished."
Blades: Eastwood, Phillips, Kelly, Whiteman, Semple, Evans, Heh (Gordon 57), Dimaio, Wright (Khan 72), De Girolamo, McDonagh (Smith 79)