in that same quest Thancred also explains that he did not actually
loose his eye at all when he materialized in the Dravanian Forelands after his trip through the Lifestream, merely injure it, hence he took to wearing a scarf over his injury while it healed. It's long since healed and he has retained his eyesight in both eyes, but he still wears the scarf over his eye because.. this is
Thancred we're talking about, being Mr Ladies Man and finding it
makes him look roguish and dangerous and
thus appealing to the ladies...
The important thing is though that Thancred's eyesight is fine, it was his aether that was altered permanently, meaning he will no longer be able to cast magic at all or even
teleport, he is effectively like a Garlean who are also incapable of manipulating their bodies' aether and thus cannot cast magic or teleport.