Haaland was diagnosed with a stress reaction in the bones and there is no information yet on when he will return. The length of the healing process really depends on how serious Haaland's injury is and which bones are injured, but Haaland's injury treatment is unlikely to require surgery that would require the player to be sidelined for a long period of time.
Haaland is a classic striker and a big change from Pep Guardiola and Manchester City after many seasons playing without any striker. The past season, his appearance made many clubs and defenders in Premier League shocked and surprised but it seems things have been changed in this season.
Clubs and defenders now more understood about Haaland and how he played. Hence they can control him better and this factor plus with absence of Kevin, caused a significant drop in Haaland performances and goals for Manchester City.
This injury will make things worse for him and I remembered that he started to get issues with injuries in second half of past season. Hope he will come back soon to make Premier League more exciting.
I think Haaland will be ready to return to the Manchester City squad in January, but of course it all depends on the progress of his injury recovery. The squad will be more complete if Doku, De Bruyne and Haaland return, they are important players who enable Manchester City to get positive results in each match.
Manchester City will very need Haaland when Champions League returns. He is one of most important piece in their attacking system and if Kevin returns too, they will get two big boosters to increase their attacking power a lot.