Like you, I think Manchester City beat Copenhagen, especially with Kevin De Bruyne back. His skills improve greatly. I'm planning for the quarterfinals and beyond. City is strong but not invincible. They'll likely beat Copenhagen, but Real Madrid and Bayern Munich will be tougher. Those games will show if City can win it all. Football is unpredictable. It's not enough to have top players like De Bruyne; it's how they play together under pressure. That will decide if they can win Champions League again
Even without Kevin De Bruyne, the match against Copenhagen should still be an easy match for Manchester City because there is a difference in the quality of the two teams, although surprises may still occur when the favorite team performs badly because they slightly underestimate their opponent but on paper Manchester City's winning percentage is very big in the first match of this knockout round so that the quarter-finals will be theirs and City having Kevin De Bryune is a big advantage, that's why it is reported that the club has proposed a new contract for the player because there are reports that there are Arab clubs who are interested in him.
When Manchester City qualify for the quarter-finals, the match will be even more difficult because the team that advances to the quarter-finals will of course be a team that is more evenly strong and that will be enough to determine City's next steps whether they can qualify for the semifinals and final or will stop.
Copenhagen in the group stage of this season's UEFA Champions League competition was very impressive and the fact that they progressed in a group were big teams like Galatasaray and Manchester United failed to make it out of the group, tells how good of a side they are and can be. Against Manchester City in the round of 16 of the competition, I expect Copenhagen to play in their full strength without fearing the world, European and English Premier League champions.
However, Pep Guardiola's Manchester City is one team that doesn't only play top football performance when one or two player of the team is around. During the start of the season, Kelvin De Bruyne was injured and was sidelined for over four months and that didn't stop Manchester City from winning games as they won all the six games they played in the group stage without De Bruyne. Copenhagen might prove to be a strong team to play against but against Manchester City in the round of 16, I expect Pep Guardiola's men to sweep them aside and progress to the next round of the competition even without the help of Kelvin De Bruyne.