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Buchanan (15), who had earlier in the season finished his debut year in the Red Bull MotoGP Rookies Cup, produced a sizzling ride to place second in the opening race of the final round of the British Talent Cup at Donington Park.
A day later, he took a big tumble while battling for fourth in the second race and suffered a wrist injury.
"I was having an absolute blast until the rear let go coming into the penultimate lap, resulting in a massive high side," Buchanan said.
"I’m gutted to end on such a low after the epic high of yesterday so will try and focus on the positives, including a PB lap time."
Buchanan said he was "battered and bruised" with a suspected scaphoid fracture in his wrist as he and father Stacey prepare to fly home to New Zealand.
"I will need more X-rays in 10 days once the injury has time to settle but not sure that’s an option in MIQ."
The injury would be a footnote to his extraordinary year on the road and his delight at finishing on the podium in race one at Donington.
“It won’t define my debut international season. And nothing will take away the high of race one on Saturday. To start 14th on the grid and then close down a 19sec gap in the pouring rain to finish second felt unreal.
“It’s that feeling that’s addictive, and I want more."
Buchanan said the first race at Donington was tricky as he did not want to push too hard before his tyres got to the right temperature in wet conditions.
He watched his Microlise Cresswell Racing team-mate, 14-year-old Irishman Casey O’Gorman, and two other riders open a large gap on the field, but he calmly got back into the race.
"I saw I was gaining by over 3sec a lap and saw the chance to get a podium and help Casey to gain points on the other riders in front.
"I got past them with two laps to go and pulled away to finish P2.
"Words cannot describe the feeling. I have never had a race like this and I will cherish this moment for the rest of my life.
“From a team perspective, that result gave Casey the championship lead because I took valuable points off his main rival, so I was proud to contribute to him taking out the overall title."
That would have been ninth had he not crashed in race two, and higher had he not missed five races due to Rookies Cup commitments.
The Southland father-and-son team return to New Zealand on October 11, and will eagerly rejoin their family after two weeks in MIQ.
Buchanan will then focus on the New Zealand race season with the Yamaha Racing Development Team.
"I’m excited to see what can be achieved at home.
“In terms of international racing, the preparation starts now for next season — physically, mentally and working hard to put together the financial resources needed to make it happen.
"But I’m excited to be coming back to Red Bull MotoGP Rookies Cup and the highly competitive race environment over here.”