Carl Froch feels Anthony Joshua grew too relaxed once Andy Ruiz Jr. replaced Jarrell Miller for what emerged as an epic upset.

According to Froch, once Joshua lost his edge to face the trash-talking Miller, it impacted his psyche and allowed Ruiz to beat him. That wouldn’t have happened, the former super middleweight champion claimed, had Joshua fought Miller in Joshua’s debut in the United States on June 1 at Madison Square Garden in New York.

England’s Joshua (22-1, 21 KOs), who was more than a 20-1 favorite the week he faced Ruiz (33-1, 22 KOs), was knocked down four times – twice apiece in the third and seventh rounds – before their scheduled 12-round fight was stopped in the seventh round.

“I think the late change was bad for AJ, in terms of getting his mental psyche up for the fight,” Froch said during Sky Sports’ recently released episode of “The Gloves Are Off,” a promotional tool for the upcoming Joshua-Ruiz rematch. “I think he was too relaxed. I think AJ would’ve took his foot off the gas and relaxed mentally. He’s not got Jarrell Miller growling at him anymore, and he’s not worried about the shove at the press conference. He’s gone. Who’ve we got in? We’ve got Ruiz. Let’s have a look at Ruiz. He looks like a physical wreck. Bodies don’t win fights. So, AJ took his foot off the gas. To answer your question, I think the late replacement was bad, and it went against AJ.”

Ruiz replaced Miller (23-0-1, 20 KOs) once Miller tested positive for three performance-enhancing drugs.

Joshua knocked down Ruiz with a left hook in the third round and appeared on his way to producing the type of knockout the odds indicated he’d record. Ruiz recovered quickly, however, hurt Joshua with a left hook in that third round, floored the former champion with a right hand and went on to pull off the biggest upset in heavyweight boxing since Hasim Rahman knocked out Lennox Lewis in the fifth round of their April 2001 bout in Brakpan, South Africa.

Ruiz, of Imperial, California, will defend his IBF, IBO, WBA and WBO titles in their immediate rematch December 7 at Diriyah Arena in Diriyah, Saudi Arabia.

DAZN will stream their 12-round rematch in the United States. Ruiz-Joshua II will headline a Sky Sports Box Office pay-per-view show in the United Kingdom (£24.95).

Keith Idec is a senior writer/columnist for BoxingScene.com. He can be reached on Twitter @Idecboxing.