Unfortunately, for him, he is the perfect sub. If a striker comes on with 20 mins left, it's usually because we are either chasing the game or out of sight so the team will be set up to attack, which is perfect for old Charlie as he is pretty much guaranteed chances. HRK doesn't have the same impact off the bench because he is not a natural centre forward and nowhere near as clinical, his hold up play, however, makes him a better starter.
For this reason I think we will see HRK continue to start with Charlie coming on.
Still need another striker though as the "Lesser Spotted Zohore" appears on the verge of extinction.