of course he's good enough, he's been the best player of the europa league as an attacking wingback last season, and also a top5 player in assists in the last few years. he didn't forget his job suddenly, he just can't do it on his own. as it's been mentioned a million times since the beginning of the season, we need more bodies in the box. even this season he's got pretty good stats:
when you see napoli attacking, that's 5-7 players in the box. with juve, it's vlahovic wrestling with 2-3 defenders, and an occasional kean or mckennie running like a headless chicken. kostic would be very useful in the attacking phase in a well functioning team.