United boss Alex Ferguson hoped Rooney may return after this Saturday's FA Cup clash with West Ham.
But Fergie said: "Wayne's still injured and it looks like going to be out another two weeks or so. It's a surprise as he was meant to be back training now.
"So he'll be out for longer than we thought and I don't think he will be fit for the Liverpool match.
"The injury is nothing serious. He just got clipped behind his knee.
"Nothing shows in the scan but he's still feeling it. Hopefully in two weeks it'll be sorted but nonetheless it's a loss."