Ian Sampson was critical of the defending on display from his Northampton side in the 2-0 defeat to Crewe.
Goals either side of half-time from Clayton Donaldson and Shaun Miller sealed the points for Alex in Town's first game for three weeks on the back of adverse weather conditions.
"I am really disappointed by the two goals we conceded and we need to rectify those mistakes very quickly," he said.
"If you get it wrong in both boxes like we did then you are always going to struggle. To be fair I didn't think we looked that rusty, we just didn't defend the two goals well enough."
Sampson also revealed that he is on the lookout for a new frontman in the January transfer window.
"We've got some decent strikers who can play at this level but I am looking for someone who can add a bit more quality and variety up front," he said.