That’s the question their supporters ought to be asking in the wake of their treatment of the Spanish international, who tomorrow reaches 100 days without playing a match.
The saga of his move to Atletico Madrid was finally resolved yesterday, with the Blues agreeing to sell him for £50m.
He had become almost the forgotten man of football, languishing in Brazil, the country of his birth, while Chelsea and Atletico Madrid argued over his future.
But surely the solution was a simple one. Chelsea and their boss Antonio Conte knew last season Costa wanted to go back to Atletico and also that he could not play for the Spanish side until their transfer embargo was lifted in January 2018.
So why didn’t Conte say something like this: “Diego, we may not see eye-to-eye but will you stay for six months and play for me - then you can go to Atletico with the blessings of me and the club.”
Instead, the ex-Juventus and Italy manager sent him with that now infamous text in what appeared to be a ‘machismo showdown’.
The pair clashed regularly last season with Costa making no secret of his desire to quit the club.
There was also rumblings of bad behaviour from Costa - including reports of him KO-ing a team-mate, believed to be Nathaniel Chalobah, in training.
Having witnessed his strange behaviour after Chelsea won the title at West Brom, joining John Terry and David Luiz in ‘kidnapping’ Conte from his press conference, then doing pull-ups when the Italian tried to talk to other journalists, he’s obviously not be the easiest player to deal with.
But this all misses the point about Costa, who last played football for Spain against Macedonia in June.
Yes, he may be a bit hyper-active - “a man child” was the description of one Blues insider - but despite all this he has been a cracking player for the Chelsea, particularly last season.
Some 52 league goals in 86 starts, including 20 in the title triumph. Even though his heart might have been elsewhere, Costa turned it on for the Blues and appeared to be an absolute nightmare to play against.
His looks - and he really does resemble a baddie from a spaghetti western - were matched by a snarling style of play which did not give Premier League defenders a minute’s piece.
You can bet the Arsenal backline were delighted not to have to face Costa again last Sunday as Chelsea struggled up front in the 0-0 draw - and failed to beat the north Londoners for the first time at home in the league for six years.
Tomorrow’s clash at Stoke would have been a match Costa would have eagerly anticipated, licking his lips at the prospect of facing Mark Hughes’ no-nonsense team.
Hard to imagine Alvaro Morato or his stand-in Michy Batshuayi having the same enthusiasm about a trip to the Bet365 Stadium.
A source who knows the inner workings of Chelsea very well admitted: “It was a big mistake from all sides. In this case there were no winners.”
You have to wonder whether without Costa, Chelsea will ever be winners again.
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TALKING of people in the game who look like they can handle themselves...
Sean Dyche at Burnley probably doesn’t get the recognition he deserves largely because of his image.
Interviewing him three years ago, he confessed anyone bumping into him would suspect he was “a nightclub bouncer”.
But looks can be deceptive and Dyche and the Clarets are enjoying an excellent start to the Premier League season having avoided instant relegation for the first time in May.
In the transfer window, Burnley spent around £30m - peanuts in today’s hyper-inflated market - and brought in £45m.
So far they have already picked up eight points, five of them away from home at Chelsea, Tottenham and Liverpool.
They are seventh and will be looking to boost their position at home to Huddersfield tomorrow.
Certainly it’s hard to fathom why Crystal Palace snubbed Dyche in the summer when it came to replacing Sam Allardyce.
Maybe, one day, someone will look a bit deeper and the big man will get a big job.
