The former heavyweight champion shocked the world when he knocked out Anthony Joshua to claim the WBA, IBF and WBO titles in June 2019.
However, he weighed in more than a stone heavier for their rematch in Saudi Arabia after months of partying and poor preparation.
As a result, he was outboxed by Joshua, lost his belts and has not fought since.
Now though he has joined up with Canelo Alvarez’s trainer Eddy Reynoso and shown off his weight loss.
Reynoso told Behind The Gloves: “It’s about 20lbs. And he also has more muscle.
“He’s not as fat as he was before. He can move his hips a lot better and that helps him move around in the ring…
“He’s working really hard.
“He’s trying to fight in April, but it’s not confirmed yet.
“The change has helped, coming into this camp. We know that he can do well and I can assure you he will be world champion again.”
Eddy Reynoso’s Stable
Andy Ruiz Jr (Former Heavyweight World Champion)
Canelo Alvarez (Four-Weight World Champion)
Oscar Valdez (Two-Weight World Champion)
Julio Cesar Martinez (Flyweight World Champion)
Ryan Garcia (Lightweight Contender)
Frank Sanchez (Heavyweight Contender)
Reynoso’s stable is now one of the strongest in boxing with multiple world champions and top contenders training at his gym.
Meanwhile, Deontay Wilder has also been linked with a spring fight, which would be his first since the rematch defeat to Tyson Fury.
Should both Ruiz Jr and Wilder get back to winning ways, many fans hope they will be matched together in a massive heavyweight clash later this year.