#4 Andreas Christensen (22 years old)

Christensen has somewhat suffered ever since David Luiz came back on to the scene, but there is still life in the Dane. He has appeared a lot in the Europa League this season, and he is yet to let Chelsea down.
His defensive work is a sign of the times. He isn't the typical brute defenders which the Premier League has seen throughout the years. Instead, he likes to contribute to attacking play whenever possible. Whether this is with short passes out from the back or running into midfield, you best believe Christensen will try it.
The Europa League is probably his level for now. It will take a lot to overcome David Luiz and Antonio Rudiger in the starting eleven. Both of them have looked impressive, so you can understand why Sarri has opted to not start Christensen.