Borussia Dortmund star Lewandowski is expected to join United in the summer.
But Michalczewski told BILD: For me, Robert Lewandowski moving to Manchester United in the summer transfer window would be a mistake. Dortmund is a really great club, with probably the best coach in the world.
"In Manchester, Lewandowski would have to deal with very strong competition. Besides, I'm not sure if the Bundesliga is so different from the Premier League.
"Lewandowski is having a very good time in Dortmund. He's still young and still has a chance to change the environment in the future. Remaining with BVB can do him good."