Why did Simeone sub him off anyway?
According to The Guardian Chelsea's considering a move for Antoine Griezmann in the summer worth £43m. Great offer for Atleti to take. Makes more room for Angel Correa as well.
That isn't a great offer. They should keep him, and thanks to their new investors, they can.
Does he have a release clause ?
Yes. All players in La Liga do.
Something in the mid 50 million euros, last I heard.
Edit: It is 60M euros.