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They had to think about density and backup for this season as this season they are not only in PL but also in CL. It is very hard for clubs to manage their squad with injuries and all when they have both CL and PL around. We either way needed some backups as centre back. We still might need one but I think as for now it is ok with what we have.
I feel that there should be rotation from time to time so that everyone gets chance and develop themselves. I like the fact that Arteta is giving some chances so I think he is managing it very well so far. I think in next game instead of playing Partey as right back and White as center it will be Kiwior and Saliba at center and White as right back.
Each coach will try their best to adjust strategy and player rotation based on the opponents they will face.
There are times when the coach rests several important players in a match and chooses a second player to play. That's what Arteta and all the other coaches do so we don't have to worry about that.
In my opinion, a coach will likely only have difficulty adjusting his strategy if he loses a key player due to injury. Many coaches depend on such players for their team's performance, but when injured, the team's performance is greatly affected. This is experienced by most teams, including even the top teams.