A CT scan unleashed a cacophony of medical alarm bells. Nicholson was strapped to a bed and eventually underwent an eight-hour operation. His surgeon, Jeremy Reynolds, described his chances of a happy ending as less than "winning the lottery with a single ticket".
Nicholson was walking freely two months later. Now he's back on the horse.
It's allowed Nicholson to sustain his livelihood as a trainer and rider, even if he has snuffed out his dream of selection for a New Zealand-record eighth Olympics.
That mark now looks set to be held solely by Mark Todd, presuming he's picked for Rio.
However, such an accolade seems insignificant after a life-threatening experience.
"I'm seeing my surgeon at the start of March for an X-ray," Nicholson said. "If he's happy, I'll start competing next month or April. However, riding is easy. Hitting the ground is the tricky bit. I don't ride as many horses and have offloaded about 15 out of 30. I still ride about eight a day in dressage or jumping."
Perhaps understandably, Cillnabradden Evo exited Nicholson's Wiltshire property on a one-way horse float, but the gelding was in good company. Mounts such as Quimbo and Mr Cruise Control, on whom Nicholson had won four-star events, have also gone.
Nicholson has kept his four-star veterans Nereo and Avebury, and continues to develop his latest hope Jet Set. He's planning for the former pair to accompany him to Badminton.
"There'd be a sense of satisfaction if I can do it. I've still got to practise my cross-country because it's too wet this time of the year. I might get there and not feel up to it, but what I'm doing at home is not worrying me. I'll begin at smaller competitions to get confidence."
Renowned physiotherapist Don Gatherer, who has worked with top jockeys, Britain's 1980 Olympic team, F1 drivers, skiers, boxers, rowers and the England and Lions rugby teams, is helping Nicholson with his rehabilitation.
"He's been doing full-on work with my neck and shoulders," Nicholson said. "I've had a harness put on my head, which is pulled tight with elastic bungy ropes.
"The idea is to do it using the more delicate muscles as part of the strengthening process. Don's contraption is also used to test neck strength for rugby front-rowers.
"It doesn't manage to pull my head off in the two-hour sessions, and I come out better than I come in. I started off pulling one bungy cord, now they've upped it to three."
Nicholson's recovery has been guided by the 52-year-old Britain's Injured Jockeys Fund, which has helped more than 1000 former riders.
He had no insurance at the time of his accident, but the success of his Wiltshire operation means his livelihood is not under threat.
"Twenty-five years ago, I opted out [of getting insured] because it was too expensive to keep up the payments," Nicholson said.
However, loyal owners are giving their star rider every chance to pull through.
"I'd like to think I've given them support, and that's being returned."
While Nicholson's physical strengthening has rebuilt, his mental resolve was never in doubt.
"When you're doing things for yourself, you need no motivation. Even heading back to gallop in the freezing cold and a howling gale feels good."
