Rob McCracken, the head trainer for Anthony Joshua, admits there is no room for error in the rematch with IBF, IBO, WBA, WBO Andy Ruiz.
Joshua will face Ruiz in a rematch on December 7th in Saudi Arabia.
Back in June, Ruiz captured all four world titles when he dropped Joshua four times to secure a seventh round stoppage.
Joshua had dropped Ruiz in the third round.
But then he charged in for the finish - and Ruiz caught him cold with a counter hook that changed the entire fight.
Joshua never recovered from the punch. He was dropped twice in the same round, and then two more times in the seventh.
"He's beaten numerous small, mobile heavyweights and he's lost to a couple of them,' McCracken said during a recent interview with the Guardian.
"Sometimes fighters are just good. Ruiz has good technical ability. He can measure the distance. Knows how to defend. Got a half-decent jab, good timing and speed. Ruiz is an underrated world-class fighter but if you've got the height and reach, like Josh, you must use it.
"The minute you don't, you're open to quick powerful hooks off these heavyweights. And Ruiz is one of the best. He’s motivated. He’s been in the best mood in camp for a while. He’s laughing and joking. He also knows that if he does everything right, he’ll win but there’s no room for error."
Joshua recently trimmed off some of the muscle from his frame to increase speed, stamina and mobility - but Ruiz is not concerned.
"I saw videos where he looked slimmer. I don't know if that's an advantage or a disadvantage, ' Ruiz told Sky Sports.
"I don't know if he will take the punches as well as he did when he weighed more."