Depends if Marquez actually thinks Yamal can be decisive and make an instant impact.It's possible but I've read club is also considering him for Barca B playoff game against Castilla which makes more sense.
Let him get more experience in meaningful games rather than playing 10-15 minutes in a dead rubber first team game where most players probably won't bother showing up anyway.
There will be more than enough time for him to get involved in the first team during preseason.
In terms of development it's always better to play with the first team instead of B team. He's only played like 20 minutes for them so not like he's some B star.