In general, Morata does well when the opposition plays an open game, and he doesn't do so well against a parked bus. He's good on the ball, has a good touch and speed, and that's why he thrives in open space and is good on the counter. That's why he did well vs CL sides that played open footy. Serie A defenses play a low line, defend in numbers, and don't offer much space, and opponents like that don't suit him very well.
Another one of his strengths is, when in form, he's a clutch striker. But now, he's off form, has low confidence, and even seems scared to shoot.
Just because he hasn't fared as well in Serie A doesn't mean he never will. We're talking about a young striker with potential, not the finished article. And he HAS shown plenty of potential and developed rapidly in the course of a year last season.
Dybala is an amazing, Pogba-level talent. Being worse than Dybala is no shame. They're also different kind of strikers: one is a CF, the other a SS.
If someone like Cavani is available and would join us, Morata should be sold instantly and without hesitation if 50+ million is offered. Without a WC replacement, there's no need to sell him.