The Latics had climbed to safety on the back of their trio of wins and Johnson had made eight changes to the side which beat Crawley at the weekend, but he felt the players let themselves down with their display on Tuesday night in the 1-0 defeat.
"We've had some proper home truths in the dressing room which is good because we'll now see who can react to that and who can't," he said.
"If you want to win a league or compete to win a league you have to show in every game more than we showed here. I was actually sitting there and all it did was confirm things that I already knew.
"I might be being a bit harsh but it was just the mindset and the power play, it was weak.
"They (Shrewsbury) weren't great but they were better than us. You take some players out of my team that are key ones and it makes a difference.
"But at the same time I would rather be honest with players as at least they know where they stand and they know what it takes.
"We have got a couple of people that have been promoted from League One in that dressing room and I got them to speak as well afterwards.
"I spoke because I've been promoted out of League One and I told them exactly what it takes - and this wasn't it."