Rodger pointed to complacency in the team after the 6-1 drubbing of Barnsley at the weekend.
"The one thing we did say was 'don't be complacent'," he told the Grimsby Telegraph.
"We flagged it up and the players were made very aware of it.
"But it took until half-time to get them going again and get them back on track and that is always the worry after such a good result on Saturday.
"People think they can just turn up and it's going to happen - that isn't what happens in football.
"You have to go back to the beginning and start again, you have to start the game right and you have to roll your sleeves up and win your personal battles. In the first-half we didn't do that."
Rodger did have praise for Town man of the match Graham Hockless after his second half showing.
"In the second-half Graham showed glimpses of what he can do and on another day he might have walked away with two goals," Rodger noted.