You have to bare in mind that firstly Lukaku is playing in a different system to what he'd be expected to do at Juve, the only reason we're playing 4-3-1-2 half the time is to accommodate Dybala better than he would fit in a 4-3-3 which Lukaku would do be expected to do well in too. I'm not saying we would be right in selling Dybala, not that it even matters now as nothing ever came of it. Fact remains that Paratici hasn't sold off any key players in the 2 transfer window's he's been in charge of and replaced them with crap. Given I'm not even counting cancelo as one of those key players since half the time he wasn't even trusted to hold his own defensively and Barzagli or some other more defensively sound RB had to come on.
I still maintain that we were right to sell cancelo given the problems he was causing us, whether in exchange for cash and Danilo is a good deal remains to be seen, for the sake of balancing the books it was the only option we had.
Another reason, perhaps more importantly, that we're not playing a 4-3-3 is we don't have wingers. Ronaldo for the left, sure, and then the right is...? DC is crippled. Bernardeshi...I don't even know what happened to him. Cuadrado sure, but it seems he's primarily a RB now (another position we're crippled at). Now we would have Lukaku, a player heavily reliant on service, and no one to really provide it with all of our weak points brought to the forefront, so we would barely be able to accommodate Lukaku there. Dybala could be shoehorned into the RW position if we really needed him to be, so at least with we get extra versatility with Dybala over Lukaku as well.
Speaking of systems, Lukaku isn't exactly known for his first touch and quick passing - things that Sarri
allegedly demands for his system. That presents a clear mismatch at the outset that should rule him out before anything else is even considered. He may not have sold any key players (more on Cancelo later), but it wasn't for lack of trying. It's clear at this point that the rumors were true and Paratici was trying to push Dybala out for Lukaku. As this was the only way Lukaku would've ended up here, it is impossible to not evaluate the deal as a whole with consideration to what both players bring to the table. With Dybala being far more technically skilled, versatile, and a year younger, it's clear who has the higher value.
Cancelo can go fuck himself. Mediocre player with a shit attitude that can dribble or put in a nice cross every now and then. As much as I don't like him, he still played 34 matches in his season here, I'd consider that "key" player territory, even if a bad one. One underperforming "key" player for another, it is what it is at the moment. Call it even, maybe Danilo can turn it around under a different coach.
But back to the main point. I like Lukaku, he's a great player with a good attitude. However, trading Dybala for him would've been a disaster. The way things transpired with this deal was the best result for all parties involved, Lukaku included, despite Paratici's best efforts to the contrary.