Joshua on Saturday makes his US debut when he defends his IBF, WBA and WBO heavyweight titles against Ruiz Jnr, who replaced the undefeated Miller after his six-month suspension ruled him out.
Ruiz Jnr has previously lost in 2016 to Joseph Parker, a fighter Joshua has already beaten and therefore encouraging a belief that his latest challenger is out of his depth.
Booth remains among the world’s leading trainers, having led David Haye to the same WBA title Joshua will be defending at New York’s Madison Square Garden, and he believes Ruiz Jr possesses the talent to test Joshua where Miller could not.
Booth told Press Association Sport: “This substitute main event is better than the original. The problem with Andy Ruiz is that he looks bad physically, but he’s a lot better than he looks.
“He’s got good hands, quick hands, and he can bang. The downside is he doesn’t move his feet too well and that could be the difference between the two of them, where Andy Ruiz can’t get into the fight because he can’t get into position.
“He needs to be able to move his head and move his feet at the same time. If he can he can find distance. But I haven’t seen him do that.
“The pressure’s on the others [Tyson Fury and Deontay Wilder] to make more of a statement than Joshua. Joshua’s already commercially the most valuable heavyweight out there. But he’s a fighter, a winner and a competitor, and he’s going to want to dominate.
“He’ll be smart with his legs, box right at the start, then quickly start levelling up on his shots.”
Booth is in New York with his talented welterweight Josh Kelly, who has been matched with American Ray Robinson in his 10th professional fight.
They arrived from the UK late on Monday evening, defying conventional wisdom that longer is needed to adjust to the time difference on the East Coast, but the meticulous trainer has revealed that what has been seen as their late arrival was instead exactly as they had planned.
“[The time difference is] only five hours so five days are more than enough,” he said. “He actually benefits. If he fights at 9pm at night, and 10.30pm is when the body starts to wind down, it’s closer to being the middle of the day for him. You want the fight in the middle of your biological clock’s day.
“This is his 10th professional fight. This is the belief I’ve got in his ability. I’d pitch Josh in with any of the top 15 [welterweight] fighters, outside of the elite of Terence Crawford or Errol Spence, now. He’s ready for any of them.
“He’s already the most gifted fighter I’ve worked with, with his understanding and execution. An undisputed champion, and multi-weight champion, is what I see him being.”