I doubt he'll be a back up player. He'll be part of the LW/CF rotation (in-game and for injuries, resting). I can see him getting just as many games / starts as Gakpo and Isak.We deprived Newcastle of the chance to sign Ekitike by signing him ourselves. He can't play for both of us. Seems pretty obvious to me.
I think it's reasonable to spend relatively less on backup players than first choices. I think £80m on a backup striker is quite a lot. Not totally against it, but it's a bit questionable for me.
EDIT. That twat Stevie beat me to the post again.