Alvarez demonstrated precisely why he is widely-regarded as the world's best pound-for-pound fighter, unleashing a vicious uppercut to the right side of Saunders' face in the eighth round of their super-middleweight title bout.
Saunders somehow remained upright but the damage was immediately apparent, with swelling developing under his right eye. While he dug deep to survive the round, his inability to see led to trainer Mark Tibbs waving off the fight.
As Alvarez celebrated winning his third title in the 168lb division – adding Saunders' WBO crown to his WBC and WBA belts – his beaten opponent, his head hidden by a white towel, departed in an ambulance to hospital.
Hearn, who promotes both fighters, said on Twitter: "Just at hospital in Dallas, @bjsaunders_ stayed in overnight. Suffered multiple fractures to orbital area and will undergo surgery this afternoon."
Saunders acquitted himself well for seven-and-a-half rounds in front of an overwhelmingly pro-Alvarez crowd of 73,126 – setting a new benchmark for the highest attendance at an indoor boxing event in the United States.
He was behind on all three judges' scorecards at the time of the stoppage but he was starting to find some rhythm as the fight progressed, using his superb lateral movement to frustrate his rival in Arlington on Saturday night.
But Alvarez, a four-division world champion, remained composed and turned the course of the fight with a single punch in the eighth round, a crouching Saunders unable to get out of the way of a full-blooded uppercut.
Alvarez (now 56-1-2, 38KOs) instinctively sensed his previously unbeaten rival was hurt, raising both hands to raucous cheers, before unloading more fearsome shots as Saunders (now 30-1, 14KOs) entered survival mode.
A forlorn Saunders walked back to his corner but after a brief deliberation with his handlers, Tibbs signalled to the referee the contest was over and the Hertfordshire southpaw bowed his head on his stool in grim acceptance.
Alvarez said afterwards: "I went back to the corner and told my trainer Eddy (Reynoso) I didn't think he could continue because when you break a cheekbone, you risk your life and you can't continue that way.
"I knew what I was doing and I knew what I did with that punch, I knew he wouldn't come back after I broke his cheekbone. I love boxing and this is what I love to do."
Alvarez now holds three of the four major world titles in the 12st division and IBF champion Caleb Plant is firmly in the Mexican superstar's sights.
Hearn said at ringside: "There is no excuse why the Caleb Plant fight shouldn't be made."