Reds manager Richie Barker has bemoaned individual errors in recent weeks and Sadler agrees they have been sloppy at the back.
"You can't legislate for some of the goals we have given away as a team," Sadler said.
"Another defensive mistake has cost us and we have to put it right, and quickly.
“The players are letting themselves down because decision-making at times is poor. Without being disrespectful, they are not top players”
Richie BarkerCrawley Town manager
"Strangely we have gone in patterns this season. It has been very odd.
"We will concede within the first 15 minutes for a spell of games, then we will concede just after half time for a spell of games and then from a set-piece for a spell. Now it is bad goals all of a sudden.
"I know, and we know as a defensive unit, that if we can not concede goals we can be the springboard for the rest of the team and we can then go on and win matches.
"It is hurting us as a back four right now."
Crawley have not scored in their last four games and Barker says there is room for improvement at both ends of the pitch - as all his players "have a flaw".
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