He has never really been at the top level he used to be when he still played for Bayern Munich. Last season he was doing alright, but remember how many goals he scored for Bayern and then I am sure Barcelona was hoping for a striker who is similar to what Harry Kane is doing for Bayern now. But Lewandowski won't get to that level. He is not the only one to blame though as I think that the Barcelona team is not strong enough. They are not a 10/10 team, they are more like a 7.5/10 team. Of course strikers have a harder time then.
However the team conditions are different, and so are the strategies used by the coach. he may be very productive in Munich with strong team support. and in Barcelona, there are still deficiencies in other positions that interfere with support for strikers in scoring goals.
Lewandowski will be let go by Barcelona I think. That's also because there are rumors that Lewandowski is asking for a salary increase. which I think must be adjusted to improve performance.
If Xavi finds another alternative in the transfer market, Barca may release Lewandowski.