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.
We also have to consider that the intensity of football in France is not as high as it used to be in England.
This means that the age of Aubameyang might not be an obstacle for him in Marseille but in England age is seriously a factor to consider.
Another thing that helps a player is having a regular playing time. Even if a play drops in form, when they have a continuous appearance they will have great chances of bouncing back to form. This cannot be said about a player that is aged, lost form and sits on the bench every match day.