2016/17 serie A was a bit of a statistical anomaly so this doesn't show the whole picture but...higuain scored 7 more non-penalty goals than mertens did this season, and their goal/minute ratio is very close(1.09 vs 0.98), and mertens spent the first two months of the season as a creator rather than the focal point of the offense AND it took the team another couple months before they clicked with Mertens as their main goalscorer.
How much of that is down to the differencez between the two seasons and how much is down to Sarri is hard to say. Higuain's goal per minute ratio was 0.73 for juve this season, while Napoli finished with more points(86 vs 82), more goals(94 vs 80!), and a better GD(+55 vs +48). Juventus played in third gear the whole season and finished with same points(91), more goals(77/75), but worse GD(+55/+50)
Btw, Morata has everything to be a beast. He just needs the right manager and environment