City won 4-1, with former Dortmund star Erling Haaland scoring for the hosts.
Afterwards, Kovac said: "We saw that City are a fantastic team. I think we started very well the first fifteen minutes we had possession and control of the game but during the first half they increased the speed and you saw the quality.
"You can't make any mistakes against these top players. We should be better, but it's a learning experience for us. We're here to learn, but also congratulations to City.
"We knew City are a top team, and it's hard to get any opportunities to score against them.
"When you get these opportunities, you must be very accurate. That was the difference between the sides. They had the same moments; Foden scored two goals. We had some good moments, but we need to continue the process."