Mbappe must be in Madrid first eleven. Garcia had the opportunity to play because Mbappe was sick. Mbappe is a goal machine and has more experience than Garcia. There is time for everyone to play in Madrid and that's why Madrid is keeping these young players. Mbappe was the highest goal scorer last season so don't underestimate him for a lad.
For now we can't say must be, because the player will eventually move to the clubs that can afford him in the current price Real Madrid indeed need player like Mbappe and at that the club is already closing in on bringing the world record breaking player, into their fold, but then we still have a lot of things to look out for before we can conclude on his moving to real Madrid and knowing what we should expect from real Madrid in this new season.