A prohibitive underdog and an nearly comically rotund physical specimen, Ruiz climbed off the canvas to drop Joshua twice in the third round, and twice more in the seventh.
The door remains open for Anthony Joshua to face Deontay Wilder in the biggest fight in world boxing - but the American's trainer fears AJ could struggle to mentally recover from his crushing defeat by Andy Ruiz Jnr.
"I'd love to say it was an early stoppage", Hearn said.
National Basketball Association star Kyrie Irving and rapper Meek Mill were among the celebrities in the house expecting Joshua to roll to a win and set up more ambitious, lucrative bouts against the likes of Tyson Fury, Deontay Wilder and a few other potential opponents. I know AJ was drawing huge numbers in the United Kingdom and that's what made it such a compelling fight but now I think after last night.
Anthony Joshua's reign as heavyweight champion of the world was brought to a crushing end on his USA debut. Respect to Andy and I'll move forward now. Andy Ruiz was the better man.
Joshua went down again in a flurry of Ruiz punches with almost two minutes left in the seventh and while the British fighter got off his knees just in time to beat the count his legs looked like jelly as he made his way to a neutral corner.
"I've got to get that armoury".
I have no idea what is in the future for Ruiz, but I think the whole world of sports could get behind him if he continues destroying his opponents.
Defying the bookies, Ruiz showed tremendous poise, power and fearlessness in systematically dismantling Joshua, the sport's biggest star globally.
Joshua said his first defeat was "just a minor setback" in his boxing career.
Wilder, owner of the WBC crown, chose to give Luis Ortiz a rematch. Wilder and Fury fought to a thrilling split-draw in December and were supposed to have the rematch earlier this spring, but the latter signing a deal with Top Rank momentarily foiled those plans.
"I am trying to make a legacy and a big name for myself". "This fight is for all my underdogs who are always counted out, all my fat boys who are never given a chance overlooked simply for their weight #snickers #joshuaruiz". I don't want people to drown in their sorrows. The British Anthony will just want to look away.
He was defending his slices of the heavyweight title - Wilder has the other one - and although he gave away more than 20 pounds to the 5-11 Ruiz, he enjoyed massive advantages in height, reach and athleticism.