Nigel Adkins admitted Saturday's home loss against fellow promotion rivals Peterborough United was a difficult one to take.
Having seen his side equalise twice through a Billy Sharp penalty and a Jack Baldwin own-goal, United were caught out by a solo effort West Ham loanee Martin Samuelsen which saw Posh snatch all three points and the Blades boss wasn't happy with the Blades defending.
He remarked: "There was a fantastic atmosphere inside Bramall Lane, but we are frustrated to have lost the game in the manner we did.
"Our preparation told us about Peterborough's threat from long-range and we conceded two goals from outside the penalty area, so that's disappointing.
"Overall our performance was very good, we moved the ball brightly, created chances and looked a threat going forward throughout the game.
"Peterborough will think they have scored three excellent goals, but we have to do more to prevent them. But we stuck in there and the supporters got right behind the players and it helped."
The Gaffer was particularly annoyed about how easily Samuelsen ghosted through his side's defence for the game's decisive strike, which came seconds after Baldwin had diverted Chris Basham's effort into his own net in front of the Kop.
Adkins added:"We had enough experience on the pitch to manage the game better. You are always at your most vulnerable when you have scored and the players know my thoughts on their winning goal.
"When we look back on the defending for their third goal, which has come from a throw-in on the halfway line, then our players will be mortified.
"This defeat has taken the wind out of our sails a little because we have been building up the momentum and the game was there to be won, especially having worked so hard to get on level terms for a second time."