I would have to have another look at the games from back then, but I think in the season 2014/2015 they had even more clean sheets in total. If I understand you correctly, you just mean out of the 22 matches played so far right? I just know that 14/15 was amazing as well as they started 22:0 goals out of 8 games. Lewandowski had a head start with scoring goals and he is still doing great but he slowed a bit down.
We don't know why Lewandowski has worse form than in early of season for Barcelona. Probably because of World Cup 2022 effects and he was more tired after that so he could not play as good as they did previous months. Though Barcelona are still playing well and dominating La Liga, clubs there likely watched videos and knew how to stop Lewandowski. When opponents defend better, Lewandowski will have less chances and score less goals.
Now we can be looking forward towards the game between Manchester United and Barcelona. Both are well prepared and will go into the match with high motivation.
The defense of Barcelona can still make the difference, but playing against Manchester United is quite different from playing against Cadiz of course.
Their defense system are like a bunker in La Liga but in a first leg match with Manchester United, they got two conceded goals. After 22 matches in La Liga, they got only 7 conceded goals but only a single match with United, they got two.
Their defense will be important for a next match in Europa League because with Bruno Fernandez and Marcus Rashford on fire, United can find Barcelona nets again.
Manchester United is a different beast. In La Liga they now feel comfortable and other teams know that it is difficult to score against them. That makes it harder for other teams to play confident against Barcelona.
Lewandowski is also among the top scorers when it comes to preparing goals. His overall statistics are awesome, but I just noticed that it declined a bit compared to the first months.
I am excited for the next el clasicos in Copa del Rey, those will all be great games. It is even more fun to see them play the final against each other because when they kick one out in the semi final it's almost certain who is going to win the final.