I think that is the point, because Barcelona is far superior when they are playing at home, so if United could get a draw in Nou Camp, then they could end up getting a win in Old Trafford a lot easily. If they lose, they could still make a come back, like lose 1-0, but then win 2-0 at home so United could still proceed with that mindset, but that doesn't mean that it could ever happen in that kind of way.
Playing at Camp Nou is never easy for guest teams. In the past, when Barcelona are big names in Champions League, I saw some clubs can challenged Barcelona at their home stadiums, got wins at their homes but later when they visited Camp Nou to play in the second leg, they failed and were beaten by Barcelona in aggregated result.
Barcelona are no longer big and dangerous like they were in the past but I believe their home stadium is still a challenge for guest teams.
I believe that we should be expecting a big fight from United on first game, they will throw everything they have and will try to keep possession of the ball and just slow the game down as much as possible to get a draw, will be very difficult but not impossible.
Both matches are big fights for both clubs but I don't know what made you believe that Manchester United can get a draw in Camp Nou and later they can win easily in Old Trafford.
Sounds like you are considering Barcelona too weak in this season?