Aubameyang is 34 years old and his quality has dropped a lot so I think it's only natural that he isn't wanted by Chelsea and indeed he has to go to a team that needs him more.
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It makes sense that Chelsea let Aubameyang go to sign on an offer from Marseille and it's not really a question of needing or not needing but sometimes other team see player from other teams have different views. even though we think that Aubameyang at Chelsea doesn't have a strong performance, Marseille thinks that this player still has hope.
and I think that Aubameyang transfer to Marseille has made a few good changes to the composition of his team performance.