Alex Nicholls scored a late winner for the Cobblers and Evans thought his team deserved a better result.
He said: "That was daylight robbery. For an hour there was only one team dominating and that was Rotherham United.
"Their keeper has made some key saves and at no stage did I think they were going to score. We were trying to force the winner at the end because we want promotion but we've got caught on the break and that was sloppy.
"It's a bitter blow to swallow but we have to take it on the chin.
"If Northampton are the benchmark then we'll be right up with them at the end of the season because we're better than them. They can celebrate, we will commiserate but we will bounce back."