Sunk cost isn't limited to money. Forget how he got here, and who deserves any blame for the situation.
Did he play at an acceptable level for a starting free safety last year? Yes. Is he looking more like the player he was in Buffalo so far this training camp/preseason? Yes. Will it carry over to the regular season? Who knows really.
But how he got here and what he cost and our expectations as fans and how they were not met really shouldn't factor into what we are seeing at the moment. Whether or not this player can help win football games is really all I care about. The rest is inconsequential in my mind.