Johnson, who took over from Jim Gannon earlier this month, told BBC Radio Cambridgeshire: "It's up to them to prove they want to be in my group.
"Some of them need to work hard to stay in my thoughts. We looked like we were already on the beach in Marbella."
Posh were 1-0 down within five minutes at the Madejski Stadium and despite an early tactical switch from a 3-5-2 formation to 4-4-2 they could not prevent a further three goals being conceded before half-time.
Despite being without influential players such as Liam Dickinson and Aaron Mclean, Johnson has admitted he will need new faces at London Road.
"It shows we're going to need a few more [additions] than I thought originally, but it only makes me more determined," he said.
Posh currently have a squad of around 40 players and Johnson wants to reduce it over the summer.
He has been promised money to spend by chairman Darragh MacAnthony, but the club's strict wage policy is likely to limit their potential outlay.
"The group of players is not good enough for the Championship, but we knew that," he said.
"I was aware it would be a big job as teams that are relegated do find it hard to turn things around."
Source: BBC Sport
Source: BBC Sport