I would say if Lewandowski can score in this match then Barcelona will win today. There is no doubt that Bayern Munich is a renowned and powerful team. But I don't see any way to call the Barcelona team weak this season. Barcelona is a very strong team and they are consistently performing well. And Barcelona's defense is now stronger than Bayern Munich's.
Both teams will want to get points and move to the top of the points table. So it will not be surprising if both teams play defensively and try to draw the match. But we will expect to see many goals in this match
Given the fact that the performance of both teams is great, and the defense is not very good (both teams regularly concede goals), I don’t think that Lewandowski’s one goal will solve anything. A lot will depend on who scores first. For example, if Bayern scores, then most likely everything will end with big defeat of Barcelona, if vice versa, then there is a chance that Barcelona, having played reliably in defense, will score a few more goals and then the game can end 3-2 in its favor.