Goals from Lee Barnard, Jose Fonte and Richard Chaplow had put Saints firmly in control, and after Aaron Mclean pulled one back for Posh, substitute Guly Do Prado scored a superb header.
Johnson said: "Southampton came out of the traps very quickly and I have to say that all sections of our team just did not perform.
"We didn't defend properly, the midfield didn't battle, and we didn't hold the ball up front. Southampton did all of that and the best team won.
"We have a few lads who are lacking confidence in themselves and not playing well as individuals, we had the first chance and if we'd scored it might have settled us down.
"But we will get on the training ground and shuffle the pack round and get a team that can compete and get results because in our last three or four games we have been poor."
Source: PA
Source: PA