Chelsea covet Thomas Muller, let Diego Costa go?

Talk about quick business! With Man United's adoration of Thomas Muller being the worst-kept secret in football of late, it appears that Chelsea are ready to make a decisive and definitive move for the Bayern Munich forward in the upcoming transfer window.
The Mirror reports that the Blues are apparently willing to let sulking/underperforming Diego Costa go in the summer in order to pave the way for a Muller move. Given that Man United failed with a reported £75 million bid for the versatile 27-year-old German in the last window, it'll take some serious digging through Roman Abramovich's pockets to entice the Bundesliga mega-club into considering a sale.