He doesn’t have a developed hold-up play and back-to-goal play. Both quite important for a striker as you spend most of your time with your back to goal as a #9 and you have one centre back up your arse and another nearby ready to cover the first one’s mistakes. You have far less space and time on the ball than on the wings. In addition, He is also not a particularly explosive guy. Oh and according to Van Gaal he prefers to play on the wing himself.
It begs the question therefore, if we’re buying, why buy a winger (who prefers to be a winger) to try and develop him as a striker and not just buy a striker? Given the added risk that this development might take a long time, or worse, result in a lesser player than the one playing on the wings.