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If Real Madrid has no problem with injured players in the defense line, most people must believe Real Madrid will win the match. Real Madrid actually performed better than Man City in all competitions. Sadly, there are 5 defenders injured now, all of them are main defenders. So, I can understand that you are very doubtful with Real Madrid chance to win the match against Man City. Real Madrid only can use Tchouaméni and Raúl Asencio for the center-backs, Mendy for the left back, and they have no right back. Well, Real Madrid defense is very worrying because of this injury problem.Even if they will play at home, it won't bring a big impact because they can't play with their ideal lineup. Some players are forced to play not in their best position, ex: Tchouaméni and Valverde. This will make the players probably to play under his performance and they can't optimize the contribution the the team.
It's difficult to make a reasonable prediction for this game. Both teams have had their weak moments this season and lost games unexpectedly. However, I think that Manchester City has had far more problems than Real Madrid. Manchester City went through a serious crisis while Real Madrid had more like some weak moments, but was able to recover quickly and win most of their games. The win rate for Manchester City in the Premier League is only 50%! I think that is quite an unprecedented performance by them in the negative sense. That means they can't win every second game, which is an impressively negative result for them.
Now the Champions League is a different competition and Manchester City will need a first good game. I think if they play in Madrid, I don't expect Manchester City to win that game and that is why the first game for them is of crucial importance.