You're not really making much sense. You said he was signed to shift merchandise.
He's nowhere near a big enough name to be primarily a merchandise selling signing. Ronaldo, yeah. Gerrard to LA, yeah.
His performances on the pitch are not "incidental" they are critical to his star quality and his merch-shifting potential. If he and United both fail they are not going to be selling many Pogba commemorative oven gloves.