Champions League matches are always different different tempo, different desire. Napoli has an excellent team and relatively not-so-difficult opponents, but still, this is the UCL and every opponent knows how to play great football. If Lukaku recovers, that would contribute a lot to the team since they are currently without a classic striker. However, the midfield with KDB is more than excellent.
That is why it is difficult for any team to achieve the title even though sometimes they play well in their respective leagues because competition in the Champions League does not only have quality players but there are other factors that also affect, such as the mentality of players, experience and the coach's own ability in implementing strategies. In the group stage it might be true because the opponents faced by Napoli are not too heavy but that does not mean they can underestimate opponents and are much better when Lukaku recovers from injury because his experience is needed by the team.