The midfield play
Now that they have got the ball into the midfield, Chelsea will be expected to rely on the 5-man midfield (deep-lying midfielder goes up) to hold possession with lots of lateral passing in the hope to find the front 3 via diagonal supply lines that negate the opposition's defensive pressure.
Once somebody like Willian or Hazard is fed, it takes a good back line to stop their deadly inward dribbles. Though we are well aware of Morata's poor form, he still has immense importance as he acts as an offensive pivot, who holds the ball down under pressure and feed it to any of Hazard/Willian/Pedro who often make darting runs in behind the defence.
Sarri's system can also take up a 2-7-1 formation, with the two attackers behind Morata dropping deep and the full-backs overlapping. An accurate cutback or a good cross can find the midfield runners to create dangerous goalscoring opportunities.
Sarri seems to encourage the unlikely candidates to venture into the box when a cross is played in. Kante's goal from the right side of the box from a cross from the left by Willian against Huddersfield is a prime example of Sarri's offensive tactics that can throw off a zonal marking system.
The 2-7-1 is a modern-day offensive Catenaccio or a Catenaccio which holds siege at the offensive goal with a single fulcrum upstream as opposed to the defensive Catenaccio of the 1950's. This takes time to build with continuous lateral possession in the final third of the pitch.