The goalkeeper wasn't included in last squad list and hasn't debuted with United yet
Víctor Valdés has not debuted as Manchester United yet. After almost two months of his signing, the former Barça player has only played 3 matches with the reserves team and he wasn't included in last squad list, for the match Manchester played against Sunderland.
Van Gaal said he had a little injury, without specifying which one
After the match, Van Gaal explained in press conference that Valdés' absence was due to "a little injury." When he was asked what kind of injury, the Dutch coach didn't want to give details and only said he had "a problem."
Valdés' physical condition is still a mystery and no-one knows if he is ready to play a high-level match. Despite it's evident that De Gea is United's starting goalkeeper, it wouldn't be strange to see Valdés playing in the Cup. But it didn't happen. We'll have to keep waiting for Valdés' debut.