Jamie Vardy scored the only goal to stun the Blues on Saturday and send Leicester City away from Stamford Bridge with a surprise 1-0 win .
Sarri was angered by Chelsea's lacklustre reaction to falling behind 10 minutes into the second half against the Foxes and knows they must improve at Vicarage Road, where his predecessor Antonio Conte suffered a damaging 4-1 defeat last season.
"I think that in this moment we have to think about us, only about our team," said the former Napoli coach.
"The match is difficult. We know we have to study this kind of opponent very well [but] I think that now the most important thing is our reaction after the [Leicester] match.
"In the match our reaction was not in the right way and now we have to think about our reaction after the match, just in order to prepare in the right way."
It means that while their fans might be enjoying some of their favourite Christmas films, Chelsea's players will be settling down to pour over their own video nasties.
"I think that I have an idea about what happened [against Leicester], so I want only a confirmation," explained Sarri.
"We will prepare a video for the individual mistakes. I want to speak with the group first of all and then I want to speak with three, four, five players. I don't know how many at the moment.
"But we have to do everything to avoid the same problem in the future. It is not easy."
"[On Friday] he started training with the group so I think he will be able to play," his head coach confirmed.