The two-time Champion Hurdle winner made a bad blunder at the second-last flight in his narrow defeat to Cornerstone Lad at Newcastle on Saturday.
A piece of wood from the hurdle became wedged in his hoof, and he finished lame, with Barry Geraghty dismounting after the line.
"The injury was so freakish you'd think it was impossible to even do it," Henderson told Racing TV's Luck On Sunday programme.
"A great wedge of wood went into his coronary band, right underneath the hoof.
"I've spoken to AP (McCoy), who is at the surgery, and I'm going down soon. In simple terms it's like a very large splinter underneath your nail, and it is very sore.
"The good news is that the coffin joint hasn't been infected, which is very important, so now it is a matter of getting the piece of wood out from under the hoof - but they will have to open the hoof to let it come out, because it's the only way.
"They are hopeful they can do it without a general anaesthetic, and then - all going well - it is a matter of how long it takes to grow the hoof back over, which will undoubtedly be some considerable time.
"The risk of infection is a worry - but the fact the coffin joint isn't infected is a big thing, because that is real danger area."
While hopes are high of a full recovery, Henderson cannot put a timescale on a return to action.
He added: "Nobody is even talking about when we might see him next - all that is being talked about is making him more comfortable, eliminate the pain, but he's in the best place.
"They can do the surgery standing up - which is good, because general anaesthetic in horses can be risky.
"If all goes well for the next 24 hours then I think we are through the worst of it, and then we can concentrate on getting the hoof to grow back, but there will be a hole which will take quite some time to heal.
"I think this season is going to be a grey area, and we probably won't know for another fortnight. We need to get him back in good shape and then will discuss the long-term future - if everything is right there's no reason he shouldn't return."
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