Colchester United manager Tony Humes says his side cannot continue to be "soft" after Tuesday's 3-2 Johnstone's Paint Trophy defeat by Northampton.
The U's, who avoided relegation on the final day last term, are yet to win a match in 2015-16 and have conceded nine goals in their last three games.
"We do the footballing and nice things very well," Humes told BBC Essex.
"But when it gets to doing the horrible things, we're not good enough at this moment in time to do what we have to."
He added: "It's players taking that decision on the pitch to stop that from happening - it's the individual mindset of players."
Despite Macauley Bonne scoring twice for Colchester at Sixfields, goals from Dominic Calvert-Lewin, Ryan Watson and Marc Richards gave the Cobblers a first-round victory.
Humes said: "We did a lot better, but again there's opportunities to be more ruthless and clinical - you've got to take your chances.
"If we're going to be as soft as we are, you have to take your chances, but first and foremost we can't go on conceding goals like that."
Humes signed George Elokobi, Matthew Briggs and Richard Brindley over the summer to strengthen his defence, and he said players need to prove their worth to the team.
He added: "We brought players in during the summer, I think it's up to them to stand up and show what they are made of.
"We strengthened the numbers in defence and it's up to them to go out and show it matters to them."