British boxer Joshua heads into the March 8th fight looking to back up his commanding stoppage win over Sweden's Otto Wallin in December as he continues his quest to become a three-time world champion.
But Warren believes Joshua will have his work cut out in a 10-round bout against the Cameroonian fighter Ngannou.
Best known as a mixed martial artist, Ngannou stunned many observers with an early knockdown of Tyson Fury, promoted by Warren, before suffering a controversial split-decision loss to the World Boxing Council heavyweight champion in October.
And Warren believes Ngannou's punching power could pose problems for another British boxer in Joshua when they meet in Riyadh.
"Joshua's come into this after his last fight against Wallin," Warren said Wednesday.
"He'll be a big favourite going in, but I know that if he gets caught on the whiskers (chin) then this fella (Ngannou) could cause a massive upset because he can whack.
"It's going to be interesting to see how he takes the fight and to see if he fights on the outside or the inside. Ngannou's game is getting on the inside. He's a big man, he's strong."
Joshua's victory over Wallin was his third win of 2023 but Warren said his two defeats by Ukraine's Oleksandr Usyk, the man who deprived him of the World Boxing Association, International Boxing Federation and World Boxing Organisation belts, were proof of an enduring vulnerability.
"The two fights to Oleksandr Usyk, we can see what they are," said Warren.
"People have seen him (Joshua) wobble a few times in fights, that happens with him. I'm not saying there's a confidence issue, but there wasn't that when he was such an emphatic fighter."
Fury and Usyk will clash for the undisputed heavyweight title in Saudi Arabia on February 17th.
The winner will be crowned the heavyweight division's first undisputed champion since Lennox Lewis, who held all the major belts from 1999 to 2000.
