Henderson's return delayed to frustration of the Blades
The 29-year-old underwent surgery during the summer and was expected to return at the start of this month.
However, Henderson will not be back until the New Year due to his recovery having slowed.
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Hide AdSpeed said: "Darius has been out since July. Although, he is in good spirits and moving okay, all he has been able to do is a bit of running.
"When you think about it, he has missed an entire pre-season so he is a long way behind the other lads.
"His recovery has taken longer than we thought and it has been frustrating because we could have done with him in the squad.
"We are desperate to get him back but I think you are looking at another four or five weeks before he is in the frame."
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Hide AdMeanwhile, Speed has distanced himself from fresh speculation linking him with the vacant manager's job with Wales.
The 41-year-old had initially ruled himself out of the running when John Toshack left the post earlier this season.
But that did not prevent the Blades' manager from again been linked with the job in the national media yesterday following a substantial bet being made on the former Leeds midfielder.
Speed, however, has been quick to distance himself from the story. He said: "I know anothing about it. I am here now, anyway, that is that."
Blades' FA Cup third-round tie against Aston Villa will take place at Bramall Lane on Saturday, January 8, kick-off 3pm.