Danny Higginbotham pulled no punches following Saturday's disappointing defeat at Portsmouth that ended the hopes of securing automatic promotion.
The experienced defender conceded the first-half showing at Fratton Park was a complete let down and insists that performance must be put right at Crewe Alexandra on Tuesday.
Speaking after the loss, that means United will have to win promotion via the play-offs, Higginbotham commented:
"Portsmouth came out of the traps quickly and let's make no bones about it, they were by far the better team and that is a big disappointment.
"We have to be honest and admit the first-half was performance was simply not good enough, in fact it was unacceptable - it is no good trying to paper over the cracks and looking for positives.
"As a group we have to address the problem straight away and only then can we put it firmly behind us."