I think he will make his debut, surely he can't defend worse than Tierney. Certainly provide competition in the left back department.
Is he more match fit than Tierney? If so he should play. I think that Tienrey just needs to get his fitness back and he will be fine BUT I am assuming that he will be as good as he was prior to the injury which may never be the case.
I agree re. Tierney. Unfortunately, the type of injury and length f absence means I can see him being not match fit for another few months