Defour, who is under contract until 2013, was heavily linked with a move to a bigger club in the summer with Liverpool, Arsenal and Manchester City reportedly interested, while Everton had a £9million bid for the Belgium midfielder turned down.
Any hope of a January transfer evaporated when the Belgium midfielder broke a bone in his foot last month but Ferguson is closely following his recovery.
The letter, which has been released to the media, reads: "Dear Steven, I have just heard about your injury and, in the name of Manchester United, I would like to wish you a full and speedy recovery.
"I am sure at the moment you must be feeling awful and that you'll have all kinds of questions.
"Steven, these are natural worries but I have to tell you that modern medicine and treatments are incredible, you have to have confidence in them.
"I'm going to follow your progress closely and I will remain in contact with your club to have news of your recovery."