I agree with Klaus here. To me, Bellerin looks a top talent, and I was generally impressed with his defending in the Emirates Cup and away at Dortmund. We lose a bit of height and are likely to be a bit more vulnerable on high balls into the box, but that's it.
I think Chambers looks a top talent as well, but I think a lot of people are too excited about him at the moment; to the best of my recollection he has had on average one howler per game so far. I don't expect Bellerin to be perfect, but I don't expect him to be worse either.
Ideally IMO, we'd use Chambers in the odd game to give Koscielny or Mertesacker a rest. I think it is a really, really dangerous strategy to play those two as much as we did last season and think they will cope for a second straight season without breaking down. Even more so after both of them being involved in the WC.