Well he was good against Southampton and Bournemouth, did well when he came on in the cup game.
I know all this you are saying, that's not really the point I am making. Let me bounce your question back at you, What has Lookman done to "deserve" a start? Purely not being Walcott?
What did gomes do to deserve a start? Dinge? Bernard? It's not about what he's done to deserve it, it's more about the fact Walcott has played himself out of the starting 11 and lookman is his understudy. Once lookman gets his chance to start then it's up to him to keep it or play someone else there.
A run of games on the bench with do the world of good for Walcott. If he can't get his starting spot back because someone is doing well then that's his fault.