I haven't seen him play but wonder if he was scouted and bought when Harris was playing Glatzel and Moore up top together as an upgrade on the former? The suggestion on Twitter was that he's best used in a two based on his small sample size at that time.
MM hasn't played that system so Watters hasn't been used much beyond some minutes against Wednesday and Luton.
He's also not played much football at league level so was potentially always thought of as a player for next year onwards rather than being immediately ready this season.