Nigel Clough described Tuesday's 2-1 home reversal against Peterborough United as 'bitterly disappointing'.
With results elsewhere going in United's favour, the Blades missed a real opportunity to claw some points back on the teams above them, after goals from Josh Payne and Conor Washington condemned United to a late defeat, despite Matt Done's opportunist opener just after the hour
A visibly frustrated Clough remarked: "When you are 1-0 up with 88 minutes on the clock, you certainly shouldn't lose the game, but we conceded two unbelievably poor goals.
"We made three defensive errors for their winning goal, we lose the initial header, we don't make the challenge on the second ball and then Bob Harris fails to clear everything out as the lad breaks through.
"It would have been bad enough drawing the game, but to lose in those circumstances is bitterly disappointing, we shouldn't be losing games like that.
The Gaffer refuted claims United sat back an invited the visitors back into the game, although he conceded the loss was made even worse by the fact that his side had missed a chance to move closer to their rivals who are above them in the league table.
He added: "We didn't take our foot off the gas, we were playing with affectively ten men at the end with Ryan Flynn struggling with his hamstring and having made our three substitutions to try and win the game there wasn't much we could do really.
"It was a bad night all round, we didn't have that spark and even the substitutes failed to influence the game as we would have hoped when they were introduced.
"Our changes were enforced, Paul Coutts was ill, Marc McNulty wasn't very good for an hour and Matty Done was shattered.
"It is a massive missed opportunity given results elsewhere, Preston have drawn the game in the 94th minute, whilst MK Dons and Swindon both lost so we could have really applied some pressure with a win, or even a point would have left us how we were with Preston.
"If the others slip up we have to be there to take advantage and we haven't done so."