The London Road club were one of the favourites for promotion and an immediate return to the Championship following relegation last season but Darren Ferguson's team are languishing in sixth place in the table.
With Posh 18 points behind second-placed Leyton Orient, the play-offs are the only way Peterborough will be returning to the second tier.
MacAnthony vented his frustration via Twitter straight after the 2-0 defeat against an under-strength Blades side on Tuesday night.
United left several of their regular first-teamers on the substitutes' bench ahead of their FA Cup quarter-final tie against Charlton on Sunday but they still proved too strong for a Posh team who barely mustered a shot on goal during the whole game.
"Another (opposition) goalkeeper who had a night off," said MacAnthony. "I remember the days when we used to score 25-yard free-kicks (like the one Ben Davies scored). Long long time ago!
"That must have been the first ever £3million pound bench in Posh's history. Another record broken. Hey ho!
"And trust me Posh fans I'm just as fed up and bored watching our club underachieve consistently and with very experienced players as well!"