Riccardo Calafiori's rapid opener set the tone in Cardiff, with the hosts extending their lead through Kai Havertz before Martin Odegaard added a third early in the second half to hand Mikel Arteta's side the season's first silverware.
Speaking in his post-match press conference, Maresca pointed to the manner of the opening goal as decisive in a game that had otherwise been closer than the scoreline suggested.
"I think a big part of the game was in the first goal we conceded," he said. "Unfortunately after 30 seconds we conceded the first one. I think the reaction was good, before they scored the second goal we had three or four chances to score and to draw the game. Unfortunately we missed the chances and then after the second goal it became more difficult."
Asked whether he was concerned about the way Manchester City had conceded, the Italian was candid about the standards he expects to set in his first campaign at the Etihad Stadium.
"In general I'm always concerned, even when we win games so imagine how that we didn't win the game," he said. "It's just the beginning, we have many things that we need to do better. I think the reaction was good after conceding the first goal and now try to reset and analyse the game and think from Tuesday about the next game."
Mateo Kovacic, who started in midfield in Maresca's first competitive match, echoed his manager's view that the result did not fully reflect the balance of play.
"Obviously, today was not our day," the Croatian said in his post-match interview on Matchday Live. "I think the decisive moments were on their side. In the boxes they were better but overall I think we didn't play as bad as it sounds. But that football. At this level the small margins decide the game."
Erling Haaland, Nico O'Reilly, Jeremy Doku and Elliot Anderson were all restored to the starting line-up having missed City's pre-season tour of Asia following their involvement at this summer's World Cup, and Maresca explained the thinking behind throwing the group straight in with limited training time together.
"We decide to start with four with just two or three sessions," he said. "The reason why was also because sometimes players prefer to start doing the proper warm up and start from the beginning. The plan was to change all of them after 45 minutes or one hour maximum, so it's a decision that you take before the game."
With City's Premier League campaign now just days away, Maresca will have a brief window to address the issues exposed in Cardiff, while Kovacic's more measured assessment suggests the dressing room believes the underlying performance offers more encouragement than the result alone.