Gillingham boss Neil Harris says his players have had a wake-up call and some need to look in the mirror.
Gills won their opening four games 1-0 to sit top of League Two, but have since lost twice - 3-0 to Colchester and 2-0 at Grimsby on Saturday.
Harris fears the Kent club's fast start might have made his players complacent.
He told BBC Radio Kent: "I think some senior players in there need to have a good look at themselves. There's no excuse for lacking heart."
Gills made a record-breaking start to the campaign with four clean sheets and 12 points, and also drew praise for their performances in ousting Southampton from the EFL Cup 3-1 and even in their battling 3-2 defeat at Premier League Luton in round two last Tuesday.
However, Harris was seething after three second-half goals at home to Colchester were followed by two first-half concessions at Blundell Park on Saturday.
He said: "I can't be any more disappointed. A lot of people travelled to support us. Our performance didn't merit their support and the money they have paid.
"I didn't see it coming. That's five really poor goals we have conceded in two halves of football. I am really angry. I've not raised my voice, but I've said some home truths. We have to respond accordingly.
"The players I picked haven't performed. I was amazed to be honest. We did not compete well enough. Particularly on set plays in both boxes. Same players making the same mistakes. I don't have to accept it.
"We set our stall out with four wins from four and expectations levels rose. I was trying to play it down. I can accept losing the odd game in a season but not back-to-back - that's not good enough.
"If anyone in that changing room thinks because we've had a good start 'we're here now, that's it' - then [this is] a massive wake-up call."