It stems from a bizarre logic where some seem to think we should have a world class player at every position - which in the process completely ignores the fact that its teamwork, not individuals that wins trophies
Speaking for myself, I'm not saying we need WC players in every position but we need more than what we have.
Our best teams usually had around 7/8 of those players. We have, what, two right now? de Gea and Pogba. Bailly can become WC, and it remains to be seen whether we keep Zlatan and, if we do, if he maintains his previous level of performance. That's, at a stretch, four. I personally wouldn't class either Mkhi or Mata as WC, but more very good squad players à la Park.
We of course have the potential of Shaw, Martial, Rashford and Pereira but we need some more players who are ready now. Especially if José isn't that high on Shaw and Martial.
I'm not saying we shouldn't be buying useful squad players as they make the team what it is as well, but our priority as of now should be going for the best we can (especially in attack) as we're severely lacking in quality. Perisic is the type of attacker we should be looking at (a wide player who keeps width and is a somewhat decent attacking threat) but I think we need better. We have enough "squad players" who can fill in out wide; it's an absolute menace of a wide player that we're lacking. I guess Martial of last year could be that player but he doesn't seem to be in José's favour.
Honestly, I'm not a fan of Perisic (yes, I've watched a fair bit of Inter due to Banega) but he is a decent player who, if signed, will have some good moments here. Just like I thought with Mkhi last year and Mata in '14.