Billy Joe Saunders went straight to hospital after suffering a suspected fractural orbital bone (eye socket) in his defeat to Canelo on Saturday night.
The previously unbeaten 31-year-old two-weight world champion was stopped after the end of round eight, in which he was smashed with a right uppercut which caused the injury.
The fight began positively for Canelo who was the more active of the pair early on.
He confused Saunders with feints before hammering home telling body shots.
However, the Brit then seized momentum and appeared to be starting to win rounds himself.
His output increased and he was having more and more success up until the eighth and final round.
In this session, Saunders staggered back after being caught with a big counter right uppercut, which caused serious eye damage, and he just survived the round before his corner ended the fight.
The scorecards after eight rounds read: 78-74, 78-74 and 77-75 - all for Canelo.
It was later announced on the DAZN broadcast that Saunders had immediately left in an ambulance.
The injury is reported to be a suspected fractured orbital bone (eye socket), though this has not yet been 100 per cent confirmed.
Saunders’ trainer Mark Tibbs told Behind The Gloves: “His eye socket was caved in and he couldn’t see.
“I didn’t get the response I wanted from him. I had to pull it.”
Pound-for-pound No.1 Alvarez, 30, knew he had done significant damage with the fight-ending blow, stating "I guessed in the corner and told my trainer Eddie Reynoso 'I broke his cheek'."
"He couldn't see," promoter Eddie Hearn told DAZN. "I spoke to [trainer] Mark Tibbs. He couldn't see and he wouldn't let him go out.
"I thought Billy boxed beautifully. Canelo is so strong, he took his time. Billy is so tricky, but he busted his eye socket. Canelo coming on strong in the back end of the fight. Too good.
"It was a great performance by Billy Joe Saunders, he came to win."
Pound-for-pound No.1 Canelo declared his intention to become Mexico's first undisputed world champion at 168lbs and Hearn plans to deliver a fight against IBF champion Caleb Plant.