Dean Saunders admitted the loss of Richard Wood had taken its toll after Crawley lost 2-1 at fellow strugglers Scunthorpe.
Centre-back Wood was recalled by Rotherham in the week, leaving the Reds short of defenders.
While Kelly Youga played well on his debut in the centre of defence, Crawley were poor as a team.
Boss Saunders said: “We didn’t do enough. We didn’t defend very well at all. There were too many things going on that don’t normally go on defensively. We have been so solid.
“The right-winger was causing us all sorts of problems.
“Losing Richard Wood in midweek took its toll.”
Despite losing, Crawley remain above the relegation zone on goal difference.
But Saunders admitted they missed a chance to boost their survival bid.
He said: “Brian Jensen made too many saves today. We don’t normally concede that many chances.
“They had chances and we hardly had any. We haven’t done enough.
“It was disappointing because it was a big game today. If we had won the game we could have put ourselves in a position where we could have been clear of it before the last game. Now it will go to the last two games.”
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