Let's say you buy a player who has almost never been injured, and then you buy him for decent amount of money while he is rated very high, and in the first game an opponent breaks his leg.He has been a terrible buy so far , injury prone and nervy performances despite him having the odd good game. He is a quality player but these injuries have stopped him for gelling with his teammates and affected his qualities for us.
Was it a terrible buy? No. It was still a great buy, but bad luck.