"Trevor Hemmings, who owns Albertas Run, was very kind to give me the opportunity to ride Kauto Star instead of Albertas Run, so thanks to Trevor for that.
"You always want to ride the best horses and he is the best horse. If he puts up anything close to his King George performances before then he'll win."
McCoy added: "It's been an amazing year having won the Grand National, and the BBC Sports Personality of the Year isn't something anyone can think about, let alone a jockey."
Fehily had stepped in to take the coveted ride when Walsh broke his leg, but then hurt his wrist at Newbury last month.
"I couldn't drag it out any longer," Fehily told 5 live. "I've made the decision and I have to live by that now. I think I'll have to have surgery to fix it.
"It's absolutely devastating - to make history would have been unbelievable to have been part of. You'd start walking to Kempton now on your hands and knees if you thought you could make it."
Nicholls paid tribute to Fehily's decision, which came early enough for the trainer to find the highest-quality replacement.
"I wasn't totally surprised but I've got a lot of respect for him, not everyone would do that and you have to take your hat off to him," Nicholls told 5 live.
And of McCoy's ability to adjust to a new mount, however celebrated, Nichols added: "He'll look at videos and he'll talk to Ruby; he'll be fine, no problem at all. It's a big pressure day and if anyone can handle that it's going to be AP [McCoy]."
Two-time Cheltenham Gold Cup winner Kauto Star is one of two horses entered in the race by champion trainer Nicholls.
Henderson has two strong challengers to Kauto Star's crown with Long Run, the current second favourite in the betting and course specialist Riverside Theatre all possibles for the Seven Barrows handler.
There are two Irish representatives in last season's Arkle winner Sizing Europe and the 2009 Arkle winner Forpadydeplasterer.
Philip Hobbs' Punchestown Gold Cup winner Planet Of Sound could turn out while Tom George's Nacarat is set to have another crack.
The 2009 runner-up Madison du Berlais and 2008 runner-up Albertas Run will attempt to go one better, while the veteran Ollie Magern completes the field.
There will be an inspection at 1400 GMT on Sunday, with the King George VI Chase now scheduled to take place at 1515 GMT on Monday.
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