Crawley conceded two late second half goals in the English League Two clash to gift Chesterfield their first win after a run of six straight defeats.
The visitors frustrated Crawley for the majority of the match before hitting the back of the net in the 77th minute through Kristian Dennis and wrapping up the result 10 minutes later courtesy of Jordan Flores.
The former Premier League star took aim at his players efforts and suggested there could be wholesale changes ahead of the clash with top of the table side Luton Town on Sunday.
"It is the first time this season I felt our players didn't deserve to take anything away from the game," Kewell said after the match.
"If they think they are going to get away with playing football only now and then and not being professional it won't stand. The players that don't want to work hard will get found out.
"If a player doesn't make the most of their opportunity then there are others waiting to come in and take it.
"I need to have a very hard think about how we play Luton because it will be a very tough game."
The loss to Chesterfield is sure to put extra pressure on Kewell who was booed by supporters after the match.
Kewell has won only four games from 14 matches this season and has the team sitting in 19th position on the ladder, only five points outside the relegation zone.