Joshua returned to the ring last month, but was beaten for the third time as a professional in his rematch against Oleksandr Usyk.
AJ put in a much-improved showing the second time around after he was comfortably beaten last year on points. He fell short again though as the Ukrainian triumphed over the distance.
Atlas has now taken aim at Joshua, claiming that the Brit has "always been overrated":
"With respect, fans in the UK love a good loser," Atlas said on his podcast.
"[Joshua] knocked out Charles Martin, who's not on anybody's list of great fighters, to win the title. He's been an overrated guy.
"I can't say it any clearer or give you any better reason for it. Who has he beat?
"I say it again, who has he beat? He's lost and he's lost by knockout. Alright, the last two to Usyk, he got hurt, he lost by decision, he showed heart.
"But he got knocked out by Ruiz who took the fight on last-minute notice.
"I really believe that Joshua has always been overrated, but he can bring in big money."
Eddie Hearn is looking to increase Joshua's activity over the next year. He and his team want him to fight in December.
Talks of the Tyson Fury fight have rekindled over the past week after 'The Gypsy King' called out his rival.
A bout at The Principality Stadium in Cardiff on December 17 is being discussed.
Though fight fans rightly have doubts whether Fury-Joshua will happen next given the hostility between both teams and the short turnaround time until December.
Atlas has previously made his prediction for the proposed Fury vs Joshua fight. He thinks Fury would have too much for his fellow Brit:
"I think Fury could stop him," Atlas predicted.
"I like AJ, he's a good fighter, he's got good talent, he's had a great comeback. I like all that.
"I'm not gonna say he has a China chin, I'm not gonna say that. But, I'm gonna say Fury has a better chin.
"And I'm gonna say he's been to dark places when he got stopped by Ruiz. He being Joshua. He went to dark places.
"Now he found a light and he lit the room up and he's back into a place that's lit. I just don't think he's that far away from a power shortage.
"I think there could be a blackout again, where his confidence [goes].
"When I say the electricity, the light, I attach it to confidence, belief, mental strength and durability.
"I think that a blackout could happen in that generator of his again. He's not far from removed from being taken back to no lights.
"So I'm going with Fury."