Stags raised their game after trailing 2-1 at the break and a Davis Keillor-Dunn brace sealed them points for the home side.
“We expected them to come at us second half,” said Challinor.
“We set up in a way that we accepted we might be a bit deep but thought we could hurt them on the counter attack.
“We had opportunities to do that but didn't take them.
“But you have to defend well and the goals were awful again. They were down to poor concentration.
“You can't concede three goals and expect to win any games of football.
“We need to have more control when we have the ball – we gave it away too cheaply.”
Challinor said he would be strengthening his squad this week before next weekend's transfer deadline.
“We have to make sure we are in a position that players out there do not feel comfortable and think they're going to play all the time,” he said.
“We may be light in terms of numbers we have got, but we have just under a week to put that right.
“Hopefully we can get a few bodies in. How many may be dictated by if anyone goes out which is still a possibility.
“It will be a busy six days for us.”