The Spireites youth team went down to a 4-1 defeat at home to their Bradford City counterparts on Saturday.
Bradford took a 2-0 lead in the first half and then added two further goals before Robbie Foster scored a late consolation for the young Spireites.
Coach Miguel Llera said: "The performance wasn't good enough from the beginning.
"We didn't play last week and the attitude shown by the players in training wasn't good enough.
"We had a comfortable win in our first league game and maybe the players were thinking it was going to be too easy this season. We have to keep our feet on the floor."
Spireites: Wharton, Walmsley, Render Wild, Emery, Holmes, Walker, Turner, Foster, Fuertado, Gibson
The youth team are next in action on Saturday (11am), at St George's Park Sheffield Graves, when they face Wigan Athletic in the Youth Alliance North Group Cup.