Please don't be afraid of the treatment because it is done under local anaesthesia and is absolutely painless. Even if you have not told the doctor please trust him with his clinical judgement that he would have evaluated the case completely including the mobility of the teeth. The mobility of the teeth caused by weakening of the Gum and bone support is usually Irreversible and cannot be corrected completely. But when the infection spreads from the root canal to the bone it causes bone loss in that area leading to mobility of the teeth which can be reversed after proper root canal treatment is done and bone healing is complete. If you delay this treatment, it might become Irreversibly damaged and it might not be possible to save it by root canal treatment also. Regards.