Kepa was furious.
Wildly gesticulating with his arms, the world's most expensive goalkeeper forcibly indicated to Sarri he would be ready to face penalties against Manchester City with the Wembley showpiece locked at 0-0 and extra time about to end.
But Sarri was determined to bring on Willy Caballero, who saved three spot kicks in the 2016 League Cup final while in City's winning side.
So Kepa, who joined Chelsea in August for €80 million (then NZ$132.5 million), simply refused to leave the field.
"Once your number goes up you have to come off and show a bit of respect," former Chelsea captain John Terry said on the Sky Sports broadcast. "Deal with that after."
Publicly humiliated with his authority undermined, Sarri was furious and initially headed down the tunnel before returning just in time for extra time to end.
In the shootout, Kepa made only one save from Leroy Sane and was beaten four times as Raheem Sterling completed a 4-3 victory on penalties to defend City's title and keep the team in contention for a quadruple.
After collecting his runners-up medal, Sarri sought to diffuse the controversy.
"It was a big misunderstanding," Sarri said. "I understood the goalkeeper had cramp and for me he was unable to go to penalties but the problem was not cramp so he was able to go to penalties."
With Kepa offering no immediate public explanation, it was left to Sarri to do the talking.
"Kepa was right but in the wrong way," Sarri said. "He was right for the motivation but not for the conduct."
The insubordination exposes the player power that has appeared to often cause problems for Chelsea managers. It leaves the 60-year-old Sarri without a trophy in his career and Kepa's defiance adds to the manager's problems with Chelsea down to sixth in the Premier League.
The incident distracted attention from how Sarri had set up his team to successfully thwart City two weeks after a 6-0 loss in the Premier League.
"Today the players played exactly the match we prepared," Sarri said. "I think I am fully in control of this situation. ... We conceded nothing to the best team in Europe."
Former Chelsea manager Jose Mourinho said seeing Sarri undercut by Kepa made him "really sad."
"I don't like that he leaves his coach and the assistant coach in a situation of great fragility," Mourinho said while covering the match in Spain for the internet streaming service DAZN.
The episode took the focus off Pep Guardiola winning his third major trophy in three seasons at City.
City did lose top spot in the Premier League on Sunday (Monday NZT) after Liverpool drew 0-0 at Manchester United. But City are only a point behind the Reds and the Abu Dhabi-owned team remain in contention for the Champions League and FA Cup as well.
"The only problem is we will miss a lot of important players for the next few weeks," Guardiola said. "Being in four competitions is so demanding, so tough. We don't know where we're going to arrive.
"[Aymeric] Laporte, Fernandinho are injured, but it's much better of course when you win."
And win calmly – without a flashpoint like Kepa defying his manager.
"I've never seen it [before]," City captain Vincent Kompany said. "I wish I could do it every now and then when I don't want to get subbed off."