I don't think Barcelona's win or loss against any other team will be enough to say that they will overthrow Manchester united or lose to them in the Europa league. It will be a very interesting match to watch cuz both clubs are doing well. Every Single player in that Match will be dangerous, especially the attackers. so I don't agree with your idea that paying more attention to Robert Lewandowsky will weaken Barcelona.
Barcelona and Manchester United are big clubs with so good players and they can decide results with a single shot. Recent forms are important but not most important factors. In fact their actual performances in those two matches will decide results. Sometimes star players can have streaks of bad performances but in big matches, they suddenly revert their forms and help their clubs win.
Like Rashford who suddenly became a very different player after World Cup 2022. He will be so important for Manchester United to beat Barcelona in coming matches between them.
Lewandowsky after a good first half of season for Barcelona tends to slow down his performance. It can impact Barcelona ability to score against Manchester United. However, they will have Dembele, Gavi and Pedri who can support Lewandowsky very well and can score goals too.