Johnson told BBC Radio Cambridgeshire: "They're around, they're under contract and will be involved in the next couple of games, but I want a smaller squad.
"They might get their way back into the squad, or they might want to look elsewhere, it's up to them."
Both Simpson, 23, and Griffiths, 24, were brought to Posh by former boss Darren Ferguson, but made their debuts under Mark Cooper.
Simpson initially arrived on loan from Histon before signing a permanent deal in January, while Griffiths joined from Dagenham in October 2009.
Posh host West Ham at London Road on Wednesday with Joe Lewis expected to be fit after missing Saturday's defeat to Olhanese.
"It was just a precaution as there's no point in risking Joe at this stage. It was a hyper-extended elbow, but he's fine now," Johnson said.
Johnson intends to give most of his squad a run-out against the Hammers and has warned fans not to read too much into his starting line-up.
"Everybody still has an excellent chance of being in the starting line-up on 7 August, it won't get serious until the Nottingham Forest game," he said.
"At this stage it's a case of mainly 45 minutes each and I think it will be a good game as long as we ensure there are two teams out there."
Gabby Zakuani will not be involved as he continues his recovery from groin surgery, while Lee Frecklington underwent minor knee surgery over the weekend.
Source: BBC Sport
Source: BBC Sport