Phil Jones is definitely out of Saturday's third-round tie at Old Trafford, and Van Gaal is worried about losing further players ahead of two Premier League away games in six days.
"Phil Jones is injured and a lot of players are in the red zone," the manager said at his pre-match press conference on Friday.
"Young (Ashley)was back in training this week. He is not injured, he is overloaded and in the red zone. More players are in the red zone and we have to decide if they can play against Sheffield United or not.
"Lingard (Jesse) has trained fully this week so he is ready."