They wish to inform friends and supporters that Adonis is now in critical condition in intensive care at the hospital, following his fight against Oleksander Gvozdyk.
"They appreciate the concern and support of everyone for Adonis".
Stevenson (29-2-1, 21KOs) entered the night as both the sport's longest-reigning and oldest champion.
Michel said he and Stevenson's family were with him and that "they want to inform friends and supporters that Adonis is now in a critical condition in intensive care in the hospital following his fight against Oleksandr Gvozdyk".
Stevenson held the WBC title from 2013 to 2018 and successfully defended the belt against Britain's Tony Bellew.
Stevenson appeared to recover, but a second volley of blows from Gvozdyk appeared to shift the momentum entirely in the Ukrainian's favour.
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Oleksandr Gvozdyk stopped Adonis Stevenson at 2:49 of the 11th round Saturday night to take the WBC light heavyweight title from the Canadian.
Canada's Stevenson was rushed to hospital after being carried out of the ring on a stretcher and the 41-year-old's promoter Yvon Michel tweeted that Stevenson is in critical condition. Griffin missed a clear 3rd round knockdown for Gvozdyk when Stevenson was felled by a right hand, and then again in the 10th - this time for the defending champ after a clear shot to the chin wobbled the visiting challenger, with only the ropes breaking his fall.
Gvozdyk, 31, was fighting in Canada for the first time. He hit Gvozdyk in the abdomen and shook him, causing Stevenson to finish the round strongly.
Stevenson was leading on two of the three judges' scorecards and was even on the third after the 10th.
Earlier, unbeaten NABF heavyweight beltholder Oscar Rivas (25-0, 17 KOs) outpointed Fabio Maldonado (26-1, 25 KOs) over ten rounds.