I have reviewed the X-ray. The 2nd molar does show decay on the side where the impacted wisdom tooth was present, and the decay appears fairly deep. Whether a simple filling will be sufficient depends on whether the nerve of the tooth is involved. If the nerve is still healthy, the tooth can be treated with a filling. RCT is not automatically required just because the decay is deep. I would recommend getting the tooth clinically tested for nerve vitality before deciding on RCT. If there is no spontaneous/night pain and the nerve responds normally, we can try to preserve the tooth with a filling. Since the wisdom tooth has already been removed, it is also important to treat the 2nd molar soon so that the decay does not progress further.