I feel net spend is very limited in terms of judging a manager who has only held the position for a short period. But if they have held the position for a long enough time to oversee the majority of the team being purchased under them, I would understand its use. Any sale (profit/loss) in that case will be based on the signings that the manager made, not based on the previous one.
The net spend is greatly affected by the sale of Lukaku, who was signed under Mourinho. If you assume that all signings are made/wanted by the manager present, should it not be Mourinho who receives plaudits for signing a player who actually maintains most of his worth? Net spend can be too greatly impacted by the previous manager when it is such a short time.
Look at the team Mourinho inherited. Who was actually worth anything at the time? Martial, Rashford (Potentially, but not much as it was less than half a season), and De Gea. Other than that, there was nobody of great worth; the players he sold were not really worth much and the other options weren't much better. (Schneiderlin, Schweinsteiger, Rooney, Carrick, Depay, Darmian, Keane, Blackett, Powell, Valdes, Januzaj, Varela, McNair).
You could argue that the choice to sell gives them the credit, but I don't think that true. If you take this argument, it would in some ways be criticising another manager for not selling. Mourinho could have sold De Gea for £60m and it would have made his net spend a whole lot better, but judging by his performances in 17/18, it would have been a bad decision. In the same way, Solskjaer has had the choice to sell Pogba for a hefty amount; no matter whether you are a fan or not
Also, another issue with this net spend is that wages are ignored. Surely, wages should be included to determine the true net spend? Our player wages this year are supposedly higher than ever, by almost £30m annually.