It is purely down to consistency with Lookman. He has the ability to change games, like he did when he came on against Spurs. However, he just doesn't produce the quality game in game out. He needs to be more 7/10s every game, rather than 5/10 and then a 9/10 every so often.
I am a huge fan of his ability, but ultimately it is down to him to up his level in training and most importantly in matches. If Silva doesn't believe he can up it from here at Everton, then perhaps a sale would be best for both parties.