The 2016 King George VI Chase winner Thistlecrack could only come fifth out of six runners at Newbury in his first race since January after a leg injury.
The Colin Tizzard-trained 11-10 favourite finished 13 lengths behind surprise 40-1 winner Beer Goggles in the Long Distance Hurdle.
Tizzard said Thistlecrack will still aim to defend his King George crown on Boxing Day.
"As long as he comes out sound we'll be going there," said the British trainer.
'This hardly boosts the prospect of him retaining the trophy' - analysis
Cornelius Lysaght, BBC Radio 5 live racing correspondent
It was disappointing not to see Thistlecrack make a triumphant return from injury, but on the plus side he was only beaten 13 lengths by horses who'd mainly had runs before this jumps season.
Blaming lack of race fitness, trainer Tizzard said that now the horse was older and heavier, maybe he took longer to get right, although in the run-up the fitness vibes were positive.
The King George remains the aim, but with principal rivals Might Bite and Bristol De Mai on a high, this hardly boosts the prospect of him retaining the trophy.