Most of the Frenchman's goals have come away from home this season.
Ancelotti said: "Anelka can show his quality when he has space and, at home, it's more difficult for him to find the space.
"It depends on the behaviour of the team during the game.
"Against United, we won't have a lot of space but, if we can move well at the right time, we can find the space to counter-attack them.
"He can show his quality."