Bournemouth's club-record eighth successive league win against Carlisle consigned the Blades to the play-offs for the second year running and Higginbotham has concerns about their form heading into the season's finale.
The Blades have won only one of their last six matches and turned in a disappointing display on Saturday at relegated Portsmouth, who cruised to a 3-0 win.
"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," Higginbotham told the club's official website.
"We have to be honest and admit the first-half performance was simply not good enough, in fact it was unacceptable. It's 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."
The club sacked Danny Wilson earlier this month in a bid to kickstart their promotion challenge, but have secured only four points from the nine available under replacement Chris Morgan.
The Blades, assured of a top-six finish, take on Crewe in their game in hand at Gresty Road on Tuesday before completing their league fixtures at home against Preston on Saturday.