City's dismal campaign in Europe's elite competition came to an underwhelming conclusion with a 1-0 defeat at Borussia Dortmund.
It left them bottom of Group D with just three points - the worst return by an English side in Champions League history - and without even the consolation of a place in the Europa League.
It had been a tough assignment for City, with Real Madrid and Ajax the other opponents in a group comprised of domestic champions, but the disappointment after such high expectations was clear.
Mancini said: "I think that we had a difficult group. It was (also difficult) last year but we had also a good team.
"We had a good team and we got only three points - this is not good for us.
"We are disappointed we didn't go into the second stage of the Champions League and we are also disappointed because we didn't stay in the Europa League.
"But it is clear the season is not finished. We have the Premier League and the FA Cup.
"I think that we need to work more now to improve this problem we have with the goals (scoring).
"What we continued to do is not enough. We need to work more."
Mancini has bemoaned the lack of goal output from his principal strikers in recent weeks and was again frustrated as they failed to find in the net at the Westfalenstadion.
Edin Dzeko twice tested Roman Weidenfeller but it was an otherwise slow-paced display and Dortmund, despite having already won the group and missing key players, carried the greater threat.
Joe Hart saved well from Ivan Perisic and Kevin Grosskreutz before Julian Schieber turned in a Jakub Blaszczykowski cross for the only goal in the 57th minute.
With Ajax losing to Real Madrid, the opportunity had been there for City to seize third place in the group but they were unable to get back into the game.
The players were reluctant to speak about their disappointment afterwards.
Hart did address TV reporters immediately after the final whistle and Scott Sinclair gave an interview to the club's website but otherwise players declined to talk to media.
There were claims last weekend that City would ban the players from speaking, to avoid being questioned about this weekend's upcoming derby against Manchester United, but that was denied.
"I live in a free country where every opinion is free," Mancini said in his press conference after last Saturday's game against Everton.
The players did have the chance to speak to general media in the post-match "mixed zone" but none were forthcoming, although it was clear the result had been a major disappointment.
Sinclair did make that apparent as he spoke to City TV.
The 23-year-old said: "It is a disappointment but now we have got to move on and concentrate on the Premier League.
"All the players have a winning mentality and every game we want to win.
"It was a tough group but I am sure when it comes round again everyone will be more aware of it and make sure we do better."
Sinclair, who has struggled to make an impression at City since his August move from Swansea, made his first start since September.
The winger insists he is happy to bide his time and wait for his opportunities.
He said: "It was great that I started the game, for my experience in the Champions League, but I am disappointed we lost the game.
"It's always going to need patience. I'm playing in a great side and I've got to be patient.
"I've got to take every training session as it comes and when I get the chance I've got to prove myself."
City now have until the weekend to get the result out of their system before facing United.
Top spot in the Barclays Premier League, as well as local pride, will be at stake in the eagerly-anticipated derby at the Etihad Stadium.
In the short term, Sinclair at least has a reunion with his girlfriend Helen Flanagan to look forward to after the actress' involvement in reality TV show 'I'm a Celebrity Get Me Out of Here'.
He said: "My girlfriend's back now so I'll see here when I get back."