Selling Lewandowski is a thing of the past - there is no point in talking about how good it would be if he stayed or something like that. Now Nagelsmann has to work with what he has - either change the game plan, taking into account the fact that Bavaria does not have a center forward, or endure until the winter transfer window and buy Ronaldo (this will be the only option in winter). I don't see any other options given the state of the market and the frugality of Bavaria.
Losing Lewandowski is indeed a big loss for Bayern Munich but I don't think this can be considered a mistake because Lewandowski really wanted to leave Bayern Munich and if Bayern did everything they could to keep Lewandowski at Munich, I think it can affect Lewandowski's performance and motivation in playing football (it could be that Bayern Munich's performance is much worse than it is now). So when an agreement has been reached, all Bayern Munich needs to do is find a player to replace Lewandowski.
There is no doubt Lewandowski was amazing player for Bayern Munich but now it's past because we all know Bayern Munich try their best for keeping him, but he was not agreed about this so now talking about him is not worth as he is doing good job for Barcelona and Bayern Munich is also not bad at all, but they are trying to give time few players for better adjustment and having better results which is surely going to work even it's going to take time but if things could not be as they wanted then surely we are going to have few changes in this team in coming days.
Their current form in Bundesliga is surely not as they were in last few years as new players are fail to convert their chances into goals which is surely a big concern for them with current coach Nagelsmann strategy is also under criticism he needs to do some changes otherwise most chances we are going to have changes in this winter with new coach as well.