The Red Devils' chief Sir Alex Ferguson confirmed the news at his Friday morning press conference ahead of the game against their Premier League opponents at Old Trafford this weekend.
Ferguson revealed the duo both did some work during this past week's warm-weather training camp in Qatar, but the game will come too soon.
"Jonny Evans and Ashley Young are still not 100% fit," he said. "We had a good week training in Qatar and it was a great time to go there - we couldn't have picked a better moment actually when you consider the weather we've had here.
"So it's worked out very well for us and hopefully we get the benefit from that. It was always the plan at some stage to do that this January. It couldn't have worked out better for us.
"Ashley Young is not far away. He trained a bit in Qatar. Not a lot, but at least he is on the road back and he has always been an important player for us.
"Jonny is just taking a bit longer than we thought. But he did a bit of training in Qatar. Again, not a lot, but he is on the way back. The bonus for us, of course, is that Phil Jones and Chris Smalling are both back. So we're not so bad in that department."
Ferguson did not mention Wayne Rooney, who it was thought picked up a slight thigh problem during training in the Middle East on Wednesday. Therefore it is hoped he will be fit to line up against the Londoners.