It would be interesting to see how City handles their games ahead.
Maybe City in this season will be able to do what the Arsenal invincible's did during their season where they went a whole season without loosing any game in the premier league. City play well and even with the ten men they were able to still not concede any goal, even with how Nottingham Forest tried. Pep is very good, his changes after the red card ensured that Forest will still not be able to do much. And I know City as a club that do not easily concede goals, so even if Nottingham forest had scored one, they would have struggled for the second goal.
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You are jumping to conclusions too early. The only serious team City have played this season is Newcastle (which are clearly playing worse than last season). City have a super easy start of the season this year, but then there will be some tough games that will determine how strong they are. All the fun will start from the 8th round where they will meet Arsenal. Then they will have 5 more difficult games (with a break at Bournemouth), so we will very soon see how strong City is without KDB.