Lewandowski is a prolific striker at Barcelona as well. However we haven't been able to see enough level of contribution by him in big matches of the Champions League. It was like he was a more different striker than his La Liga performance. He wasn't having issues against weaker opponents in the Champions League. But he was a failure when it comes to facing teams like Bayern Munich in the group stage. He will need to concentrate more on that kind of matches from now on as those ones are critical for making it to beyond the group stage.
Barcelona actually need to try new things in the Champions League from now on. The strategies don't work good enough as we have seen for two seasons here.
Even in La Liga, Lewandowski did not show the required level in my opinion - 23 goals in a season is ridiculous numbers after what we saw from Ronaldo/Benzema/Messi. And if I'm not mistaken in La Liga, he is top 1 in unused scoring chances.
The arrival of Gundogan is a major boost for Barcelona, but if Xavi continues to use his old dull strategies that have not worked for a long time, then this is unlikely to somehow affect the results of Barcelona and their ceiling in the Champions League will be 1/8 - 1/4 finals.