Here, Press Association Sport's Matt McGeehan addresses them.
Kepa missed last Thursday's Europa League match with Malmo with a hamstring problem but returned at Wembley. He twice required treatment late in extra-time, with the match heading for penalties, so Sarri planned for his substitution. Willy Caballero, who saved three penalties for City in their 2016 League Cup win, was readied to go on, but Kepa declined to go off. It sparked confusion and angered Sarri.
Why did it happen?
Sarri, Kepa and David Luiz, the closest player to Kepa throughout the incident, were 'on message' afterwards, all insisting it was a misunderstanding, not insubordination. The explanations brought a cynical response as the images suggested the latter. Plus, the fact Sarri did not haul Kepa off to show his authority suggests all is not well. Kepa is the world's most expensive goalkeeper at £71.6million and Sarri's position is precarious, but Luiz dismissed suggestions it was indicative of players holding the power at Chelsea.
What was the initial reaction?
Under International Football Association Board rule three, "if a player who is to be replaced refuses to leave, play continues". England captain turned broadcaster Gary Lineker tweeted his surprise at the "extraordinary" incident. And former Chelsea skipper John Terry, working as a pundit for Sky Sports on the game, said: "Once your number goes up you have to come off and show a bit of respect."
Has this happened before?
There is precedent for this – and in a League Cup final. Former Manchester United goalkeeper Les Sealey continued despite a badly gashed leg in a 1-0 loss to Sheffield Wednesday in 1991, but continued as United did not have a substitute goalkeeper. In the Premier League in April 2017, while at Swansea, Lukasz Fabianski was injured in the 81st minute, declined a change in the 86th minute and conceded three goals afterwards as a 1-0 lead turned into a 3-1 loss to Tottenham.
Why is it especially significant for Chelsea?
Chelsea's history suggests players hold the influence at Stamford Bridge. The hire-and-fire policy of owner Roman Abramovich shows managers have little shelf life and players have previously exploited it. This is not the same group as the one which defied Andre Villas Boas or masterminded a Champions League win under Roberto Di Matteo, when the senior players were said to be more influential than the interim manager. But the incident suggested something of a leadership vacuum.
What will happen next?
Chelsea play Tottenham on Wednesday in the Premier League in a match crucial for Sarri's future. Lose and the Blues will slip further adrift of the Champions League qualification places. It appears impossible to jettison the world's most expensive goalkeeper, so it seems Kepa is likely to play. If he is omitted, whatever the explanation, it will continue the furore for days to come.