Brian Laws was upset with Scunthorpe's second-half defending after letting slip a 2-0 lead when going down 3-2 at AFC Wimbledon.
After Sam Winnall's opener, Irons keeper Sam Slocombe plunged to his right to keep out a Charlie Sheringham penalty and Chris Iwelumo headed Scunthorpe further ahead before the break.
However, first Michael Smith and then Harry Pell were allowed the freedom of the box to score before Chris Arthur's late killer blow.
"We didn't defend well as a team for the whole game and we were stretched," said Laws. "Even though we took a two-goal lead they were in control for much of the game.
"After they scored the first goal it was clear we became completely dismantled as a team. We know that teams will have their moments but it's very disappointing as you don't want to throw a two-goal lead away.
"It was a very open game but we could have killed it off in the second half as we had chances but that first goal gave them a lift and they caused us problems.
"If we get the third goal its game over but we didn't and if you don't defend well you put yourself under pressure. We got deeper and deeper and we were disjointed. We didn't work hard enough defensively and they were able to break us down.
"Every goal we concede is disappointing. We had enough bodies in and around them for them not to score but we didn't get a foot in and that's not good enough."