Marley fractured a bone in his neck as well as damaging several ribs after he was thrown to the ground when the ill-fated Red Art clipped the heels of horses in front of him in the Ladbrokes Mobile Handicap.
Harley was taken to East Surrey Hospital in nearby Redhill before being transferred to St George's Hospital in Tooting for further treatment. He is expected to remain there for a few more days. Red Art suffered fatal injuries in the fall.
Harley is stable jockey to Newmarket trainer Marco Botti, who said: "Obviously he's going to be out for a couple of months.
"I think he has a fracture of one of the bones in his neck. It's not bad and it doesn't need surgery, but he will be out for six weeks and then he will have another scan.
"He's fractured a few ribs as well. All in all, in a way, he was lucky because it was an horrible fall. It could have been much worse.
"He's talking and it's just a matter of time, about a couple of months. I think he will be in hospital for a few days."
Paul Struthers, chief executive of the Professional Jockeys' Association, reports Harley's injuries to be less serious than was initially feared.
He tweeted: "Brief update on Martin Harley - he remains in St George's but following further assessment last night he is not as serious as first feared.
"I've been unable to speak to Martin this morning as his mobile has run out & without his consent can't go into detail. Further update later."