Many observers felt Carragher's challenge warranted a straight red card rather than the yellow received from referee Phil Dowd. Former England defender Carragher made a personal apology to Nani immediately after the game.
The Portuguese winger injured his groin in November and also missed last summer's World Cup after picking up an injury to his collarbone shortly before the start of the tournament in South Africa.
With crucial games coming up for Ferguson's team, Nani's absence is another big blow with 10 first-team members likely to be unavailable for Saturday's match with the Gunners in the FA Cup at Old Trafford.
And with a growing injury list and a Football Association
charge of improper conduct
hanging over him for comments made after the Chelsea loss, it has been a testing period for the United manager.
"I have sympathy for myself. I didn't have an easy week myself," he said, five days after imposing a media blackout following the Liverpool defeat.
However, with the campaign approaching an exciting climax, the 69-year-old Scot added: "You have to look forward. We have big opportunities at the club and there are a lot of challenges ahead for the rest of the season.
"It is terrific to be involved in the middle of March. Every game you play is of importance. We have Tuesday's game against Marseille and Bolton in a league game next weekend. It goes on and on.
"That is why you get to March and hope you are involved in all these things. We are."
Ferguson also revealed centre-half Rio Ferdinand (calf) and South Korean midfielder Park Ji-sung (hamstring) needed more time in the treatment room.
There was better news, though, as their Ecuador winger Antonio Valencia is closing in on a return after breaking his ankle in the Champions League clash at home to Rangers last September.
"Antonio is difficult to assess because he has been out for such a long time," said Ferguson.
"He has been training very well and is taking part in a few practice matches between the players. It is deciding when we can put him into the cut and thrust of a game. It won't be Saturday. Hopefully it will be quite soon."
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