Ryan Lowe, Nicky Ajose and Damien Mozika eased Bury into a 3-0 half time lead. Ajose grabbed his second to make it 4-0 before Shaun Harrad scored two consolations for the struggling Cobblers.
Johnson said: "We're so disappointed with our first-half performance, we were very poor and at times it was embarrassing.
"Having to change our keeper didn't help because it meant I had to use a substitute so early in the game and if I could have used 10 substitutes at half-time I would have done!
"The players are making the wrong decisions at crucial times and that's the only reason I can give for our first-half display. We are allowing opponents to get into us too easily but at least we didn't capitulate in the second half.
"I could walk out but that wouldn't help the situation. You sometimes question yourself in these situations but the players have got to believe they can stay up and we've all got to do everything we can to make sure we stay clear of the relegation zone.
"The only way to get through it is to work even harder."
Source: PA
Source: PA