Desmond's wherewithal didn't come to him the first time he woke up after defeating the dragon.
He was fully conscious many times but he could hardly remember those moments. For days, he woke up screaming in pain and being put back to sleep immediately after.
There was finally a time when he was sedated enough by the king's enchanting magic that he spoke calmly. He could answer questions despite the pain that took over his entire being.
The healers confessed he was likely too damaged to wield a sword again. Whether it be because his arm had healed too tightly against his body and he had no range of motion, or because he had lost a bit of flexibility, the outlook of ever using a sword with the same force as before was slim.