Brahim is unable to compete with other players in his position. Madrid is a team with many players with good performance quality. When you can't compete in the squad, you have to patiently sit on the bench.
Brahim is still young, he can also become an asset for Madrid if he can improve his performance and convince the coach. if strong to compete then will survive. but if not then it's best to leave to play at another club that does allow for better playing time.
Actually, difficult for Brahim Diaz get regular position in Real Madrid midfielder position after impressive performance from Jude Bellingham, not only with him but also last three matches in La Liga Luka Modric is not first choose and how possibilities for Brahim Diaz will get more minutes playing. I don't have ideas with Brahim Diaz want return back to Real Madrid because he was been first choose in AC Milan and get regular position, if won't spent more in the bench better he ask to Madrid management open loan option for him to get more minutes playing with other teams, back to AC Milan looks realistic for him get back regular position than spent in the bench with Real Madrid.
Brahim Diaz only played one of three matches for Real Madrid in La Liga but as reserve player against Almeria and I don't sure on upcoming match he will get chance for playing although start in the bench.