Qualifying from the group phase to play in 16 rounds is enough for Leipzig, they don't need to struggle to beat Manchester City because City is indeed the strongest team at the moment and hard to beat, as long as they can beat the other two teams both home and away then they have enough points to be in second place and with Manchester City playing in Top 16.
Leipzig lost Gvardiol in this summer and with his importance for their defense, they will have weaker defense in this season. It has bigger impact in Champions League where they have to face with strong clubs in Europe like Manchester City than in Bundesliga where except Bayern Munich, Leipzig are not weaker than other clubs too much.
Manchester City is lucky with this draw because, throughout the group phase, they can focus more on continuing to win many matches in the English Premier League, or at least they can rotate players well.
As defending champions Manchester City will fight to reach the final, but when the draw for round 16 will also be quite decisive in my opinion, if they get another easy draw it will really help them, and favorite teams like Manchester City often get this convenience.
It is a lucky draw for Manchester City as it can help them to save a little energy for knock out phase as well as avoid injuries and suspensions on important players because of card quota. However, to defend their title, they have to play well in knock out phase and even with best players they have, without suspension, it's still a very challenging task for them.
Again in Champions League history, only Real Madrid succeeded to defend their Champions League crown.