Mancini is concerned his side's battle to win the championship could be badly hampered by an acute shortage of players.
Yaya Toure will be missing until the middle of February after the Ivory Coast refused a request to delay the midfielder's departure for African Nations Cup duty.
It has heightened a shortage of players that may result in City not naming a full complement of substitutes against Manchester United on Sunday and leaves Mancini to conclude he needs extra bodies.
"This could be the first championship for many years and we need to do everything we can to win it," said the City manager.
"United and Chelsea have won trophies for many years. So they are quiet. They understand that they can play without pressure. For us it's different.
"I said two months ago that we could have a big problem in January because we have to play big games without Yaya."
The problem for City's owners is not finding the cash, it is proving to UEFA they are serious about meeting the strict Financial Fair Play regulations that began this season and which the Blues are virtually certain to fail given the vast scale of their losses.
Only by reducing them dramatically will City's hierarchy hope to avoid punishment, which means Mancini may have to sell before be can buy.
To that end, the departure of Carlos Tevez cannot come soon enough, although any movement with AC Milan and Inter, who have also expressed an interest, is proving slow.
"It's better that we sell first," said Mancini. "Maybe then we can then take in other players."
Indeed, Toure could miss up to five Premier League games, in addition to Sunday's FA Cup tie, the fourth round should City get through and the two-legged Carling Cup semi-final with Liverpool.
However, Mancini is also acutely aware that with Tevez remaining in Argentina, City have effectively been operating one striker down for the past three months.
"If we have strikers injured then we need to buy another," said Mancini.
"Because (Edin) Dzeko has a problem in his knee, at the moment I have one striker and two midfielders."