#2 The absence of Diego Costa
In June of 2017, Costa had a fallout with manager Antonio Conte and since then, things have gone downhill for Chelsea. The Spaniard didn't play even a single game for Chelsea this season, before transferring to his old club Atletico Madrid in January.
Costa is blessed with the capability of holding up play and battling out with defenders, before sending the ball into the back of the net with a deft finish.
He was a perfect fit for the physical Premier League, scoring 52 goals in 89 appearances for Chelsea over the course of three seasons.
He has found his goal-scoring touch back at Atletico, and has scored five goals and made two assists since joining them earlier this year.
Alvaro Morata may be the perfect No.9, but Chelsea could have done much better with Costa right now.