City missed a number of good opportunities at the Stadium of
Light, but failed to find the net as they slipped seven points behind
Manchester United in the title race.
"It is impossible that our strikers didn't score," said Mancini. "It is a big problem at this moment.
"We are too soft in attack and too soft when we have chances and shoot."
City have scored 34 goals in 19 matches in the Premier League this season - 14 fewer than neighbours United,
who beat Newcastle 4-3 on Wednesday,
have managed in as many games.
Perhaps more tellingly, they have only scored 12 in nine games on their travels, compared to United's 22 from 10.
Sergio Aguero, Yaya Toure, David Silva and Edin Dzeko
were all denied by Sunderland goalkeeper Simon Mignolet on a frustrating
afternoon for the champions.
"We didn't deserve to lose. We dominated, so I don't know how it is possible that we lost this game," said Mancini.
"Last year, we scored one, two, three goals every game. Now when we have chances, we are too soft and this is not good.
"We should be very strong when we have chances,
sometimes we take one extra touch and maybe think 'Okay, we will score
next time.' But football is not like that.
"We have a fantastic attitude because we want to win
right up until the last second but we can't recover every game. We need
to score before that, we had five or six chances in the first half. It's
incredible."
Despite City falling seven points behind United at the
halfway stage of the season, Mancini says his team can still defend
their title.
"United score a lot of goals, but they also concede more than us," he said.
"It depends on us because the season is long and we can
recover, no problem, but we need to change something in our team
because we cannot continue like this."
Mancini was also upset with referee Kevin Friend, who
he believes missed a foul by Craig Gardner on Pablo Zabaleta in the
build-up to Sunderland's goal - scored by former City winger Adam
Johnson.
"Maybe Zabaleta deserved a red card?" Mancini said
sarcastically. "I don't think the referee and linesman had a good
performance today.
"It is impossible that they didn't see a foul like that."