Northampton boss Aidy Boothroyd was unhappy with the way his side surrendered their unbeaten home record against Gillingham.
The table-topping Gills, beaten for the first time in League Two by Rochdale on Saturday, maintained their 100% record on the road with a 2-1 success at Sixfields on Tuesday night.
Goals in either half from Deon Burton and Jack Payne saw Martin Allen's side pick up three more points despite Adebayo Akinfenwa pulling one back for the hosts late on.
"We cannot use the excuse of injuries for that performance because that simply wasn't good enough," said Boothroyd. "I want to be at the top end of the table - the players here are good enough. I'm not frightened by the teams up there, although Gillingham are a very good side.
"You put a squad together at the start of the season and you are going to get injuries, but we've been undone by one huge punt down the middle of the pitch for their opening goal. You see that sort of goal on a park on a Sunday not in professional football. It was difficult for me to understand because my central defenders have been outstanding for me, but not in this game.
"It was an appalling mistake, especially to give a goal away just before half-time. These things happen and you have to battle back from them. But I don't think we did battle back into it. That was a very average performance and there is still a lot of work to be done."