hmmmm, if hes not very versatile (which is a hidden stat only viewable with the editor) then he may lose a position if he becomes accomplished or natural in another.
however as you havent played him as a striker then im not sure. you havent been training him to play as a striker have you?
i once trained c.ronaldo to natural centre forward and he kept his previous natural right wing positon. but im not sure what to say about this.
are you SURE he had natural Left wing, cos iv just looked only about a month into a game and he has only natural in striker? unless you bought him and he definately was then idk what to say really. iv also trained buonanotte to nautral right wing and ke kept his nautral left but this was on 09.