Rotating a team is a good one, but for me it's best done when your team has established a reasonable point gap at the top of the league. When there is no much points gap at the top of the league it's best to go with your most trusted players who can get the result for you. People are singing praises for Flick now because he has won against Mallorca, but against last Palmas when the rotation didn't worked as expected, he was heavily criticised.
This is one reason I don't criticize coaches that much, especially those with limited time and resources at a club, like Flick. Barcelona's losses this season are Flick's fault but I can't blame him too much because he is in a challenging position where he has to make decisions that affect the team.
Lewandoski needed the rest. He has played a lot of games and he also has more tight games to play so resting him was not a very bad idea. It was a big risk, but a necessary risk. If he had played and probably gotten injured, Flick would have been blamed for not giving him a rest.
Players like Casado, Pedri and Olmo didn't finish the game which is good also. Barcelona cant afford to drop any more point this month and one way to do that is to keep the player fit and fresh.