City won Tuesday's Champions League encounter 2-0 thanks to goals from Erling Haaland and Bernardo Silva.
Marcelino said afterwards: "They outplayed us in the first half. Those first two chances that didn't result in goals had a big impact on us, causing us to doubt ourselves.
"We struggled after their goal, but just when we were playing our best, they scored their second from a move that I think could have been avoided. We played very well in the second half and reacted well.
"We have to bear in mind that we were playing against a great team like Manchester City, but I'm very proud of the team's performance in the second half.
"We came out determined, showing resolve, sticking together, and we created enough clear chances to have scored. I'm optimistic; I see the team I want to see, and I can see that they're excited."