Actually, there's not much difference between Chelsea and Manchester United. I also still find it difficult to know their main problem, although Chelsea were really doing well in the early stages of the season, but right now they have a decline in performance, which is still unknown to many fans, although Manchester United are still better but still unable to keep their winning streak, which is affecting them. Amorim needs to work harder to keep the team consistent.
Furthermore, Manchester United will be facing City and Nottingham Forest, as you said, and this is going to be a difficult one for them, seriously, because Manchester City are doing better than Manchester United currently.
Current standings position, Chelsea have better performance than Manchester United in this season get difficulty break to the top fourth standings or securing the UEFA Champion League spot for next season.
Chelsea indeed have inconsistent performance but large opportunity with the top fourth position and have opportunity will return back to UEFA Champion League for next season.
After defeating by Arsenal last match at London derby match, Chelsea have another London derby match for upcoming match after international break. I think facing Tottenham Hotspur should be not difficult match for Chelsea and get back three points if won't drop from the top fourth standings, for Manchester United reach the Champion League spot standings almost difficult after having many points left with Chelsea or Manchester City recently at the top 5th standings.