Nigel Clough described Saturday's 2-1 home defeat to Crewe Alexandra as 'unbelievably disappointing'.
Speaking in the aftermath of the loss at Bramall Lane, following Ryan Colclough's last gasp winner for the Alex, Clough acknowledged his side were below their best.
He remarked: "We were good in parts, but not good enough to win the game and to lose in the manner we did is unbelievably disappointing.
"We huffed and puffed but our decision making and final ball was poor when we got ourselves into some promising positions. We were also wasteful from set-pieces.
"We looked mentally and physically tired after our efforts this week and again we didn't help ourselves by falling behind early on.
"Crewe had their chances in the first-half and caught us on the break as we looked to try and get back into the game, but we ere in control for long periods in the second-half and just couldn't get our noses in front."
The Blades boss refused to criticise the application of his players, but conceded the Blades failed to pick up from where they had left off against Scunthorpe United in midweek.
Clough concluded: "It can be difficult to break sides down sometimes, we managed have managed to frustrate teams like QPR, Tottenham, West Ham and Southampton this season, but occasionally it will happen to us when teams come and sit behind the ball.
"Several players didn't reproduce their performances from the Scunthorpe victory, but sometimes for whatever reason it just doesn't happen, it certainly wasn't for the lack of trying."