Agree with you, Arne Slot should maintain the condition of several main players so that in the last 16 they can still perform impressively, fielding second-string players is not bad thing considering that it can also be very useful to add experience to shape the mentality of the second-string players.
So far, whether in the Premier League or UCL, they have shown very perfect quality of performance, don't let problems like what Manchester City and Real Madrid experienced with player crises and getting series of difficulties to achieve victory, it just that we don't know how their ambitions and goals are because Arne Slot could also try to perform well in both competitions.
Football just like every other sport is unpredictable. The addition of Mbappe to Real Madrid made most fans think that they will be outstanding both in the La Liga and the Champions League. But the reverse is now the case because they might not automatically make it to the round of sixteen if they don't win the next two games. But Barcelona which I thought would struggle based on their meagre and new coaching crew is now doing well in the domestic league and continental tournament.
It will not be a bad move for Barca coach Arne Slot to use reserved players for the next games since they have already qualified. It will help to protect important players from injury and also reduce fatigue. Young or second-fold players also need this game to help them gain experience and prepare them for the future.