Mancini started with two out-and-out strikers in Lisbon, but neither Simone Zaza nor Ciro Immobile made much of an impact as Portugal dominated.
Portugal scored the only goal of the game through Andre Silva early in the second half, with the 1-0 win the least Fernando Santos' men deserved.
The result leaves Italy with a single point from two Nations League games, and Mancini accepts their problem lies in attack.
"Unfortunately, we are making errors and need to try to reduce them," Mancini told Rai Sport after the game.
"But the boys gave everything they had. We have to score goals in order to win, so we must find new solutions and do more in attack.
"In the first half we had a lot more of the ball with the two forwards, as at least they can play between themselves up there, so it can be an important option."
Mancini made a total of nine changes from the team which drew 1-1 with Poland last time out, but he defended his selections, adamant the only way Italy will improve is if he gives players chances.
"We have to let them [the young players] grow, there is nothing else to it," he said.
"When a player is young and he is just starting out at this level, he is going to have some problems, we knew that was going to happen.
"When our fitness levels get better, that will definitely be a positive, but the main issue we need to resolve is not scoring goals."