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Ruiz Jr hadn't been in the ring since being badly beaten by Anthony Joshua in their rematch in December 2019, when there was lots of criticism of the Mexican's preparation.
The former world champion was actually knocked down in the opening round but managed to recover to control the fight later on.
Arreola continued to put in a great effort, even when Ruiz got on top, and took it all the way to the end of the 12th round, to take it to the judges scorecards.
The judges all gave the fight to the only man to have beaten Joshua, with the 31-year-old winning wide at 117-110, 118-109 and 118-109 on the cards.
Those scores really didn't please Arreola who went on an expletive rant. "I respect the judges, but today they like a butterface... Everything is pretty but their face.
"Honestly, man. Did he win? Fine. But don't tell me you're only gonna give me two or three rounds. F*** that," the loser began in an expletive, NSFW, rant.
The 40-year-old, who has twice fought and lost fights for a world heavyweight title, wasn't the only heavyweight who scored an early knockdown to be unhappy with the judges on Saturday.
Dereck Chisora sent Joseph Parker to the mat briefly on Saturday, at the A.O Arena in Manchester, also in the first round but went on to lose a split decision to the New Zealander.
The British boxer won 115-113 on one card but lost 115-113 and 116-111 on the other two, despite being the far more aggressive fighter throughout.
Following the loss, Chisora also had a go at the judges, saying, "I was putting in more power punches, inside work, but I can't get upset anymore," Chisora told Sky Sports Box Office.
"But you know what, I won't let them slow me down. No way. I will not let them slow me down. I will go again.
"They want to see me retire, but I'm not retiring yet, forget that."
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