Canada right-back Johnston and Japanese defender Kobayahsi have been training with the Parkhead club over the past few weeks since joining from CF Montreal and Vissel Kobe respectively, but they are unable to feature in matches until after the transfer window opens on January 1.
It means both players could be handed their debuts at Ibrox on January 2, but latest new signing Tomoki Iwata will note feature as he has not yet arrived in Scotland after his initial loan move from Yokahama F.Marinos was confirmed on Friday.
“Tomoki is in Japan so there is zero chance he will play,” the Celtic manager said. “With Alistair and Kobayashi, they are available like everyone else.
“We also still have to wait until international clearance comes through. Although they are registered with us, clearance doesn’t come through until after January 1. If their clearances come through, they are available and will be in consideration.”