#1 Kalidou Koulibaly
Napoli's defensive stalwart has become a major target for many clubs in need of defensive reinforcement this season and rightly so. Koulibaly has been a rock at the back so far and he is having his best season of his playing career thus far.
He has made 1.7 tackles, 1.2 interceptions, 3.9 clearances and 1 block per 90 in all competitions and also has been dribbled past only 0.5 times this season. With United's squad sorely lacking in defense this season, he can cause an instant impact to this United squad. If he is utilized properly, he can be vital for them.
It may cost United a world record fee for Napoli to sell him but if he does join, he will be head and shoulders above every defender right now in the United squad and help them and De Gea, who has been suspect this season, gain his confidence back. He can be the defensive leader they have missed all these years.