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No one doubts the quality and contribution of Luka Modric to Real Madrid and he has proven this so far. I think there are many other young players in Ancelotti's squad who have developed a lot and maybe that is the reason Luka Modric plays more from the bench and is no longer a starter. This is normal because if Luka Modric does not renew his contract, it will not cause the club's performance to decline because they have found the right replacement.
The arrival of Belingham and several other midfielders has also really helped Ancelotti's current team, so that they managed to occupy first place in the standings. Bellingham can play with any midfield that Real Madrid has and I don't think this will affect his performance in contributing to the team.
Luka Modric just has to accept that this might be his last season with Real Madrid. The coach has started to rotate regularly which means other players get more playing minutes than Modric, but of course Ancelotti will not take a big risk by substituting Modric for some tough matches such as against Barcelona at the end of the month.
Bellingham is not in the same role as Modric, but Bellingham has certainly helped a lot this season. Kross and Modric are unlikely to stay much longer at Real Madrid and that's why I find them on the bench more often than not starting most games.