Mancini rated his side's performance as one of the worst he had seen before admitting that he doubts their chances of overhauling the gap between themselves and United.
"It was a really poor performance," said the Italian. "Southampton were better than us. We conceded two goals which I didn't think a team like us could concede. I've never seen our team concede goals like this.
"In the end we made two incredible mistakes. The performance was really poor. We played this game with only two or three players. Three versus 11 was really difficult.
"I was really disappointed with my players today. When we perform like today I'm very angry. The players were away on international duty - that's not an excuse, but it is a reason. Some of our players came back on Friday and we played today. This made it difficult. In the end we played badly.
"It'll be very difficult to win the league. I use to be always be optimistic and want to be in this moment, but it's difficult. I think we now have 10% [chance of winning the title]. I don't think United will drop 12 points."
United will play their game in hand against Everton tomorrow.