Johnny Fisher believes that Anthony Joshua needs to be wary over Daniel Dubois when the pair fight.
The two heavyweights meet for the IBF title on September 21, live on DAZN .
The clash is a chance for Dubois to become champion for the first time in his career, while for Joshua it would mark the third time.
Fisher will be fighting sooner than that, against Alen Babic on July 6 - also on DAZN - and speaking to Sky Sports , he cautioned Fisher over his sparring partner, Dubois.
He said: "I think it will be a knockout. I think Dubois is going to bring the heat and put it on Joshua. That's what he knows, that's his way. He's an aggressive fighter but he's calculating that aggression.
"Dubois takes no prisoners in sparring. You've got to be on it. It's a firefight when you go in with Dubois.
"We've seen how good Joshua is with the counter right hand," he continued.
"If he [Dubois] comes in, if he steps in too much, Joshua's going to counter him with the right hand, like he did with [Francis] Ngannou.
"I know Ngannou's not on that level but [Joshua] has got a good straight right hand that's lethal. Charles Martin he's done it against. We've seen it many times. That's the danger for Dubois."
Live on DAZN: Anthony Joshua vs. Daniel Dubois on Sept. 21. Click here for details .