Both Barcelona and Manchester United have strong squads. There are many star players in their squad. But despite having a lot of talented players, the two teams are still not stable.
If compared to Man United, Barcelona may have a better squad and performance. However, as you said, both are sometimes inconsistent enough with their performance. Moreover with some problems that Man United faced previously. After being eliminated from the UCL, I am sure that Lewandowski was stressed enough because he cannot bring Barcelona to continue to the next stage. That is why I am sure that in this UEFA, he will do his best to make his new club shine again. However, Xavi has learned many things after their failure at UCL.
Lewandowski feel disappointed caused by barcelona got kicked out from UCL yet he can do nothing about that's why he must accept the truth. MU gonna be a very tough opponent for barcelona. MU has been on fire in the curabao cup and marcus rashford may become the one who will be a thread for barcelona. Xavi will be losing some players like de jong that may be going to the MU. Im sure that MU will able to defeat barcelona in the first leg of europa league. I think that both have a capable squad to defeat one to each other. MU gonna be on fire against barcelona.