Kepa, 24, refused to be substituted deep into injury time despite appearing to struggle with an injury, prompting angry reactions from Sarri and his technical bench including assistant coach Gianfranco Zola.
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But now it appears there was a good reason behind Sarri was desperate to bring on the former Manchester City goalkeeper for the penalties.
The Argentine has a stunning penalty record under his belt since moving to Europe from Boca Juniors in 2004.
Caballero has faced a total of 38 spot-kicks and has saved 11 of them with his recent one coming in last season’s FA Cup when he denied Hull City’s David Meyler as Chelsea cruised to a comfortable 4-0 win.
Caballero, 37, has saved penalties from the likes of Riyad Mahrez, Eden Hazard, Falcao, Neymar, Oscar, Youssed El Arabi, Ruben Castro, Cristiano Ronaldo, Jorge Molina and David Lopez.
City captain Vincent Kompany confirmed Caballero’s penalty prowess after the game saying: “I know how good Willy Caballero is on penalties, last time we won it and he won it for us so I didn’t want him to come on! It didn’t happen in the end.”