He won't be. We did not spend all of that money on Sesko to play Zirkzee as our starting striker, particularly when you consider that Zirkzee himself says he is not a striker and his career record of scoring about half as many goals as Sesko from a similar number of games should tell you who is more effective in front of goal.
I do think Zirkzee is a talented player. I don't see a natural position for him in this side however and again he is not and will never be a starting number 9 for a top team, it is not what he wants nor does it play to his strengths.