Manchester City are weak in comparison to last season when they won the league and the Champions League, but this season it is difficult for them to score goals easily since De Bruyne go to injury
Man City isn't weak. They just doesn't perform as well as in the previous season. But it doesn't mean they are a weak team. Check again the group G, Man City is leading the group. They won all the matches so far, so how can you judge Man City is weak now? Sure, when De Bruyne is injured, Man City can't play in their best shape. But they still showed us that they have enough power to compete until the final round.
I believe that Manchester City will not win the Premier League this season because they are not demonstrating that they can win the league when they play against Brighton
How do you know this? You can't conclude it from a single match only.