City came from behind to lead 2-1 through a Mateo Kovacic brace before Doku struck as the hosts eventually won 3-2.
The Belgium winger later said: "We are very happy with the result. We knew already in advance that this was going to be a difficult game. They are a good team with good players.
"They scored a very good goal in the beginning and that made the game even harder. On their counter attacks they were very, very dangerous. In the end we handled the game well. Kova got two nice goals and I scored as well so we are very happy."
On his goal: "It is a different goal for me because I'm not the type of player that shoots a lot from distance. Most of the time my goals are more from dribbles and shooting from close, so this was a bit different. Honestly I don't care how I score, as long as I help the team I am happy.
"When I took the shot I felt that I had hit the ball well and then it was just about hoping that I had put it in the right direction. I did and it went in so I am happy."
On making a big impact from the bench, he added: "When you are on the bench it is not about being a spectator, you look at your position and try to see what you can do if you come in. You want to come on and make the difference.
"I saw that they were playing in a back five and the defender on my side was a winger that was playing defensively, so I knew with my one versus one qualities that there would be a lot of space. No attacker likes to defend so I tried to make it as difficult as possible, get the ball and go at him."