"We made too many mistakes, but the guys gave everything," said Mancini.
"We need to find a solution to score goals because they do not win games without scoring, that's our main problem right now.
"Portugal won first, so it's the favorite, but I think it's still open, you never know in football."
Silva scored in the third minute of the second half with the AC Milan striker at Sevilla collecting a cross from Bruma to beat his former Milan team-mate Gianluigi Donnarumma on Italy's goal.
"I have some friends in the Italian national team, but there are no friends in the field," Silva said.
"In general, we were the best team at night and we deserved the victory."
Italy was lucky enough to keep its goal clear in the first half, thanks in large part to Donnarumma, as Portugal dictated a game at Estadio Da Luz.
The effort of Bernardo Silva was cleared by Alessio Romagnoli with a shot of Bruma on the crossbar.
Italy fought with Ciro Immobile and Simone Zaza threatening just before the break.
Mancini had marked the changes with an experimental side with seven new players as starter: midfielder Jorginho and Donnarumma, the only pair to be retained from the draw with Poland.
& # 39; Italy takes risks & # 39;
In the same lineup 4-3-3, the Lazio Immobile forward intervened for Mario Balotelli, who was harshly criticized for his performance against Poland before suffering a muscular problem.
Manuel Lazzari of SPAL made his international debut in the right of defense.
Federico Chiesa and Andre Silva missed the opportunities early, while a Zaza movie about Inmobile was erased in the box.
William Carvalho shot from distance and Jorginho's volley was cleared.
Portugal caught the winner shortly after the resumption through Silva with Bernardo Silva arriving at a second solo to clear Donnarumma.
Italy struggled to create opportunities, apart from a Zaza header that widened to 12 minutes from time, while Portugal won their ninth consecutive home win.
The four times world champions now run the risk of being relegated from their League of Nations group after not also qualifying for the 2018 FIFA World Cup.
But Mancini defended his experimental side.
"We do not like to lose, but it is also true that we have to take risks and gain experience in these games, otherwise it is difficult to move quickly," said the former Manchester City and Inter Milan coach.
"We must help young people grow, since we have no alternatives.
"We would like to finish first in the group, we never know in football, we'll see what happens, our initial goal is to qualify for Europeans."
In the group of three teams, Portugal will play in Poland on October 11, and Italy will face the Poles again on October 15.
And the coach of Portugal, Fernando Santos, ruled out talking about now being the favorites.
"Do we have a foot in the final? We need both feet there, because standing on one foot you risk falling."