#4 Speed
The job of a defender in an attacking unit like Manchester United is pretty tough. For once, defenders are expected to be the first line of attack and they have to venture forward to increase the numbers in the midfield as well as create an option to keep the ball moving. Unfortunately, this opens up a lot of space in the back, which the defenders have to cover up very fast during an opposition counter attack.
At Manchester United, this covering up has been an issue and it has been holding Solskjaer's midfielders back. Like van Dijk at Liverpool, Koulibaly is lightning fast on the field and can cover up the ground in an instant. His speed is an asset for a man who is so tall, but it provides a wonderful fail-safe for the attackers in his team.
Koulibaly can match speed with almost all attackers around him and his understanding of the game is superb, which allows him to be strong while dealing with opposition counter. The Senegalese also uses his speed to great effect when he is the last man defending and that makes him a great asset.
As such, with Koulibaly in the team, Manchester United can go forward with the confidence that he has their back during the opposition counter.