Ferguson believes that the absence of Terry will have a vital bearing on Sunday's potentially decisive encounter at Chelsea.
He said: "Players who have been at the club for a number of years like Terry, [Ashley] Cole, [Frank] Lampard and [Petr] Cech give them stability.
"Continuity with their kind of experience must help the dressing room. Terry will be a loss for them because he has been such a dominating figure. He is their captain, he has experience and he can be a ruthless defender. His presence a lot of the time was the most important thing for them. You will always miss a player like that."