Chris Morgan was frank in his assessment of the Blades performance in Saturday's 3-0 defeat to Portsmouth at Fratton Park, which means United can no longer win automatic promotion from League One.
Speaking after seeing his side concede three goals in 11 first-half minutes, Morgan conceded the opening 45 minutes was 'unacceptable'.
He remarked: "The first-half performance in particular is very hard to take, it was a million miles off how we performed against Swindon and Brentford and it must be addressed.
"We weren't at the races first-half and I will have a look at things in that respect, perhaps I should have freshened things up a bit.
"We need to look at possible changes, but we won't make a knee jerk reaction because we still have a very good group of players.
"Of course we are dissapointed, even after we changed things at the break and got more balls into the Portsmouth box, we still didn't get enough bodies in the box and put them under pressure.
"We have now got two fixtures remaining and it is imperative we finish strongly and get mentally prepared for the play-offs.
Morgan continued to analyse the display: "We lost the game between the ears, Portsmouth's desire and application to their work was much higher than ours and that isn't acceptable, I told the players as much.
"We showed tremendous character against both Swindon and Brentford, but ultimately we have paid the price for a sloppy performance today, although Portsmouth deserve a fair amount of credit.
"To be 3-0 down after 35 minutes is criminal at any level and we know we must improve to win promotion via the play-offs."