That situation with Mertens was about the same as the goal, where Frenkie marked no one and was aware of nothing, even after the rebound. Busquets reacted well in time but fell while trying to turn, shit happens.
Frenkie played a decent game in his passing statistical wise, he did find a few good passes and he is seldom failing at finding a team mate most of the time. He is a very talented player, especially technically. The problem with his game, especially from this position, is more deep and I can't explain it using statistics. You have to be aware that two identical passes has never been made in the game of football, hence, you need to use your eye and your brain when judging a player, especially in that deep playmaking role. Watching Frenkie and his movement, timing and decisions, is to me a bit like listening to a singer singing out of tune, while watching Busquets give me the same sense of completion and joy as listening to something by Bach. I thought the game under Frenkie was too much on the outside, too safe and unthreatening, I was constantly a bit disappointed in his positioning. A few times, by chance, he arrived with the ball at his feet facing forwards after a backwards pass, and from there he is patient and could spot players a few times in between the lines, but Napoli were aware of these players as the progression was pretty slow and obvious, it didn't really lead to much. A few times he could arrive with runs from deep, that's when he threatened the most, but not really so great in my opinion to have the lone dmc do these runs too much as it leaves gaps for counters. I thought a better position for him was the one from last game, as I wrote in my previous post.
It is very much about taste and style and so on though. I am strengthened in my belief that Frenkie shouldn't play DMC after yesterdays game, but maybe there is a place for him to do it once in a while anyway when Busquets could use some rest, and it should be taken into account that Frenkie didn't play the role that much.