The 41-year-old was forced to leave the ring on a stretcher with promoter Yvon Michel later reporting his fighter had been taken to intensive care in Quebec.
He spoke only to confirm Stevenson's condition and added that there would be no further comment at this time.
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It is only the second defeat in Stevenson’s career, after a run of seventeen fights without tasting defeat, including a 2013 victory over the recently retired Tony Bellew.
But he had little answer to the Gvozdyk, who becomes a world champion in just his sixteenth fight.
Stevenson was making his 10th title defence since winning the belt against Chad Dawson in 2013. Stevenson was ahead on two of the judges' card and tied on the third when he was stopped.
Stevenson fell early, in the third round, after getting hit by Gvozdyk. But referee Michael Griffin said it was a slip. Gvozdyk, however, was the more aggressive of the two and it showed the rest of the round.
In the sixth round, Stevenson threw more punches to the body, something that he rarely does. He hit Gvozdyk in the abdomen and shook him, and Stevenson finished the round strongly.
Stevenson was taken to hospital immediately after the fight (AP)
Stevenson controlled the centre of the ring for part of the match, but in the 10th round, Gvozdyk finished with a series of blows that seemed to motivate him for the 11th round.
Gvozdyk continued with his series of flurries that got Stevenson in the ropes in the middle of the 11th, before sending the Canadian to the canvas with three solid right punches.
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