Aden Flint's early opener was equalised by ex-Stag Ryan Stirk only to see Jordan Bowery scuff a volley in off the post for the winner.
“There was plenty there for me to be pleased with but we are frustrated to end up on the wrong side of the result,” he said.
“We knew they would be a threat from set plays. Aden Flint is a big old lump and he is fantastic in the air. He has done that time and time again and we were unable to stop him which gave them that momentum.
“The the winner was a very stuffy goal to concede.
“It was frustrating as I thought we were on top in the second half. Nothing really dropped for us.
“We came out really well. We switched to a 4-3-3 just before half-time and it that seemed to alter the momentum of the game and got us back on the front foot.”
Ex-Stag Danny Johnson was booed all afternoon but failed to get the goal he wanted on his return.
“The group really wanted him to have an afternoon to really remember for obvious reasons but unfortunately it didn't happen for him today,” said Sadler.
“We were short on bodies today – down to the bare bones – and we were not able to name a full bench. So we are looking forward to getting players back.
“I think we have a really good group and the makings of a team that can be very good in this division. We need to get some bodies back and improve things in January.”