#5 Shaky back-line

Chelsea missed out on signing Koulibaly, and they will rue that mistake. Chelsea have Antonio Rudiger, David Luiz, Cesar Azpilicueta, Andreas Christensen and captain Gary Cahill on their books.
Gary Cahill is way past his prime, and should have been shipped out. David Luiz and Antonio Rudiger are both inconsistent. David Luiz is better offensively than he is defensively, and is a better defensive midfielder or sweeper than center back.
It is up to Sarri to mould him into a Koulibaly-like player, as he capable of playing out from the back. Rudiger is solid, but he seems to have trouble adjusting to the pace of the Premier League.
The very underrated Azpilicueta is a natural fullback, but he is the most dependable center back at the moment. It will be interesting to see how Sarri handles this, and if he will change his preferred formation with a back four to a back three.
23-year-old Kepa was signed by Chelsea for a world record fee. The €80 million man made only 54 appearances for Athletic Bilbao, but is seen as a top talent in his home country. Even the most hardcore Chelsea fan will admit that Chelsea have taken a huge gamble.
The pressure will be massive, considering the stature of the club, his transfer fee, and demanding coach. There is no doubt he will be a world class goalkeeper in a few years, but it is expected he will have a difficult time just as De Gea had when he came to Manchester United.
Rob Green looks like he was signed just to make up the numbers with regards the homegrown department. Willy Caballero is coming off a dismal World Cup, and is short on confidence. The pressure is on Kepa to deliver.