Mendes would still get a big payday selling Costa to Arsenal though. If Chelsea don't take him there really aren't other suitors besides us. In the end these guys are all supremely self-interested, I don't think Mendes is butchering a deal for Mourinho's sake, unless there's something amorous between them or something.
Costa could still stay at Atletico but his wages are apparently very low, hard to see him turning down a big contract. Simeone seems confident he can replace him too.