Lambert, in Florida with the rest of the squad, feels Wednesday will be the acid test for the injured ankle.
The Scotland internationalist will resume training on Wednesday after being sidelined for almost three months with an ankle injury.
Team-mate Alan Stubbs has come through abdominal surgery as he continues his fight against cancer.
The defender will remain in hospital for a week while his progress is monitored.
Lambert, in Florida with the rest of the squad, feels Wednesday will be the acid test for the injured ankle.
If he feels no ill-effects he can prepare to challenge for a first team place, but he is concerned that if the ankle is not ready, the injury could be exacerbated.
"Wednesday is D-day for me. If I can twist and turn that will be a clear indication the injury has healed because I haven't been able to do that up until now," said Lambert.
"Obviously I'm anxious. If I feel anything during training I'll know I've taken a massive step back in my recovery.
"My real fear is that I go all the way back to square one. It's like taking a step into the unknown.
"Whatever happens there's no chance of me risking the injury in a friendly against American opposition."
Pathology report
Lambert has indicated that he will be leaving Parkhead when his contract ends in 18 months' time.
He may also find it difficult to walk straight back into the first team even when he is 100% fit with Neil Lennon, Alan Thompson and Stilian Petrov all on good form.
Stubbs will remain in the high dependency ward at the Beatson Oncology Unit for a few more days.
A pathology report within the next fortnight will reveal whether further chemotherapy will be required.
Even if further treatment is not required, Stubbs will still be sidelined for up to another four months as he recuperates from the surgery.