If you're a top dribbler but can't hit a barn door, then you've found your weakness and know what you need to work on. It's very specific, and it should be much easier to get better. If you just isn't good enough in any part of your game, then you got one hell of a job in front of you. Like for Pereira, god knows what he's good at, and for me he'll never be good enough for a side challenging for the title in any of the top leagues. "Pereira, go be smarter, play the ball faster, learn to read the game better, shot better, make good runs!". That won't work. For Mason Greenwood, it seems to be easier; "Mason, go gain some muscle weight, continue making those runs and you'll get there. We know you can't expect much service with the support from Pereira and JLingz, but they wont be here next season!".
We can't fix all the problems we have in our team at once, but it helps to narrow the problems down. In October/November, we couldn't create chances. We're better at that now. We even managed to fix that without chopping Pereira. God knows how.
December was a half-decent month. Now in January, we've not managed to set enough of our chances. Martial doesn't look like he'll get there for us. James looks like he needs a rest. Pereira doesn't look like he has the qualities you'd expect from a United player.