The Gunners captain has reportedly scrapped negotiations over a new contract at the Emirates.
Aubameyang is Arsenal’s top scorer with 13 goals this season, but has been unable to halt their slump to 11th in the Premier League table.
And Brassell insists Gunners chiefs must cash in on the 30-year-old if a £70million or £80million offer is made.
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He told talkSPORT’s Trans Europe Express: “If I was in Arsenal’s position, I would absolutely take the money for Aubameyang because I think at the moment Arsenal have got a very ill-fitting unbalanced squad.
“This really is getting into the sort of time where this is their last chance to sell him.
“You’re looking at a 30-year-old player who relies pretty heavily on his pace, even though, he’s bringing back more of his all-round game for the first time since he was at Saint-Etienne.
“He was just a pure goalscorer at Dortmund and in his first bit at Arsenal, so we’re seeing a bit more of an all-round Aubameyang, which suggests maybe he will age a bit better than some have expected.