Kompany and Nigel de Jong used to play together at HSV midfield. Not that often, but they did. Otamendi and Jones aint that much better, if at all, than Smalling and Bailly. However, Cities midfield and attack is much, much better than Uniteds. Both on paper and chemistry wise. They are not as exposed as United CDs are. Thats the difference.
Van Gaals system was hold the ball, don't let our defense be exposed. It wasn't pretty, but it made our defenders look much better than they are. Smalling is a good defender. Not that good on the ball. Jones is a decent defender, but he's both injury and error prone. Bailly is so raw. He can be best in the world, however I always doubt defenders who rely on their physical attributes more than on their wits. If he can "evolve" like Ferdinand did, United will have one **** of a defender.
Get new fullbacks who are capable on the ball, either make Lukaku/Sanchez/Martial work or replace most likely Martial with right winger who's a good football before everything.
Lukaku is good, Sanchez will be great this season, he looks at his best. Pogba can do everything. United got Fred who complements both Matic and Pogba. Mourinho is doing a good job at identifying targets. Give credit when due. This is last chance for many players in the club. Smalling, Shaw, Martial are all on the verge of being sold or being key figures. Well something like that.
Rojo, Darmian, Jones can all go and United will feel nothing. But you need extra defender than. Toby M (*cough* spiderman) or Toby A. I wasn't serious about Alfie Mawson the other day, but even he would fit United better than Jones/Smalling/Rojo. Question still remains on Lindeloff. Can he adapt and be the CD United needs?