The Italian's concern is likely to raise a few eyebrows among those who have watched the world's richest club spend hundreds of millions of pounds putting together an outfit to mount a serious assault on the Premier League title.
The league leaders have been depleted this month by the departures of Yaya and Kolo Toure, who are on Africa Cup of Nations duty, while captain Vincent Kompany has been handed a four-match ban after a red card against Manchester United on Sunday.
"All the people think we have a lot of players," Mancini told a news conference ahead of Wednesday's League Cup semi-final first leg against Liverpool. "But we have 19 plus keepers. It is not a big squad.
"If we are unlucky and we have three or four players injured, like at this moment, we can have a problem. The club knows this."
Conscious of the need to comply with UEFA's financial fair play rules which come into full effect in 2013/14 and which aim to stop reckless spending on wages and transfers, City have spoken of moving towards more sustainable spending levels.
Splashing the cash has yielded rewards with the club winning their first major trophy in 35 years with victory in last season's FA Cup final.
"I said four weeks ago that December and January would be crucial months," said Mancini.
"Until three weeks ago we didn't have any injuries. Now we have a problem.
"We have players injured and others who went to the African Nations Cup at a crucial moment. I am not happy about this but we can't do anything about it."
