There is a lot of time left until the end of the summer transfer window and this is in Endrik's favor. He can recover and have time to pass the medical examination for a transfer/lease. I think he has no chance of getting into the main team next season, he should now choose the option where he will play in the starting lineup and get real experience.
The problem is that he can play as a striker only. CF or SS. Only if he starts adapting to playing for example as a winger then he might have more playing time.
Otherwise yeah, it is unrealistic for Endrick to cut Mbappe from the starting lineup of course.
Better to loan him then.Competition in Endrick's position is a little more difficult because for strikers now there is clearly Mbappe not to mention Madrid's targetman now prefers Garcia especially with his slick performance in the Club World Cup. The winger position is also not easy as several players can be deployed in this position including Arda and Rodrygo and now that Mastantuono is confirmed to join and he can also be played in this position, it becomes even more difficult for Endrick in the end.
Loaning him out or even selling him even if Madrid have to take a loss I think would be a good solution rather than having Endrick in the squad but unused due to other options.