Here is the same thought some people put on the players like Vidal and Naingolann also to regard them as good deals or at least not bad deals for us: they only pick few matches/goals from them and make a conclusion that they "helped us" achieve our goals. But here is the bigger picture that they exclude: these players were quite a non factor in a larger part of season. If they actually did their job properly, we weren't struggling so much to achieve what we did at the end.
If we're talking about someone who is among the lowest earners and hardly anyone consider them among the best players in the squad, you wouldn't put too much expectations on them. But if you're among the highest earners in the squad and the club paid a considerable amount of money to get you here, then having a few goals and good matches throughout the season is by no means enough. Lukaku's second spell here was a failure. His injuries, poor form, the money we paid for him and missing out on Dybala because of him hurt us way more than a few decisive moments he had at the end of season. There is no logical way to sugarcoat this.