Nigel Adkins felt Saturday's defeat at Port Vale was 'three points lost' and admitted his side need to improve.
Speaking after seeing first-half goals from Byron Moore and Uche Ikpeazu prove to be enough for the Valiants, despite Matt Done's first of goal of the season deep into stoppage-time, Adkins conceded the Blades were below par at Vale Park.
He remarked: "We are obviously very disappointed with the result and performance. We started positively and things looked promising, but we didn't move the ball quickly enough.
"Then we concede a poor opening goal and the goal on half-time is another turning point. Straight after the break Billy (Sharp) is denied by a brilliant save, if that goes in who knows.
"We had to make a couple of changes with Edgar and Flynn coming off injured, but with half an hour to go we have put four strikers up there and gone gung ho to try and force the game but we didn't utilise that to our advantage."
Adkins was particularly frustrated having felt his side held the upper hand prior to Moore's opener.
The Gaffer added: "We have told the players so many times about being assertive on the field, the second goal comes from a throw-in and we are making life hard for ourselves, this was three points lost.
"I sat in the stand for the first 15 minutes and on such a big pitch with the likes of Coutts, Baxter and Flynn out there it was a game we should have dominated, but as always the first goal in crucial.
"We can't keep struggling with the physical aspect of the game, we have to show an aggression in a controlled manner, there is certainly work to do."