I agree with you that Lewandowski was actually not that interesting in yesterday's match even though he gave him chance to win, but without yesterday penalty Lewandowski would not have been able to score a goal even though he tried and I call it luck for Lewandowski.
After Xavi return I think it will have a big influence on Barca performance but it seems true what you say that Xavi needs a lot of time to get inspired on the best strategy for the club he coach because Barca is currently only performing with players with currently no player which can be relied on even though Lewandowski can be used as a mainstay player but for me the results are not optimal.
I know this is a tough task for Barca and Xavi as a coach has to be responsible for making the club stronger every season and this season has been like very tough season for Xavi.
I'm yet to see the best of Lewandowski in Barcelona, he's not played up to the standard he did at Bayern Munich that made Barcelona bring him into their Squad, this is a player that was known for scoring numerous goals in a single season, getting hat-tricks in several matches but at Barcelona he's not scored like that even with enough chances created for him.
He has a lot of good quality players built around him so I expect alot from a player of such calibre to deliver his team against lesser clubs, well thanks to his penalty goal that made them win Osasuna and I believe the goal he scored would increase his morale to perform better in their next match and start scoring more goals as expected by him, if Lewandowski fails to improve before the winter window it would leave Xavi with no option but to seek another alternative in that position.
This season hasn't favoured Lewandoski because he hasn't score much goals in this season. Out of four matches he has only scored one,and a penalty that he played yesterday to make it two,compared to his performance last season. He did very great in Bayern and I think that he hasn't really understood Barca playing pattern or their bond is not yet strong. It is normal that some players don't perform too well when they change leagues. I am still hoping to see the best of him and I will also love to see more goals from him during his stay in Barca.