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.
I also agree that Mbappe must remain a main player at Real Madrid. Because he is indeed a very great player and it was proven that last season he could become the top scorer in La Liga. So it is certain that Mbappe is included in the main squad at Real Madrid. But I am sure that after Mbappe recovers from his injury, he will definitely be immediately appointed as a main player by Xabi as the coach of Real Madrid. Because for now Mbappe is injured, so he cannot participate in playing with Real Madrid in the Club World Cup which is currently entering the last 8. And besides that, maybe until this competition is over, Mbappe will most likely be absent from Real Madrid. Because according to the news circulating at this time, Mbappe has stomach flu and with that, Mbappe's return time cannot be predicted. So with that, Mbappe will most likely be absent during this competition.