Bookman essentially needs to replicate his performances off the bench from the start. Its a very different thing. Some players find it very difficult to come off the bench and get the flow of a game. He doesn't struggle to do so, which is good. However it's different when you start, the pace is faster, players are pinpointing you etc.
I have no issue with him as the superb for now. You are right about end product though, he needs better numbers and assists. He played well today though and well in the derby off the bench, so there is obviously ability there.