Brook will be competing in his first world title fight against the American, but despite acknowledging the champion's power, Hearn is adamant that the Yorkshireman will prevail in California.
Hearn told Sky Sports News: "He's in a great place physically and mentally, and I'm very confident about this fight. I just feel like he's going to be too big and strong for Porter. I know Porter is big himself and looked devastating against Paulie Malignaggi, but I just feel like it's Kell Brook's time.
"I feel like he's got so many more gears to give and levels to go, and mentally and physically, he's in such a good place. His weight is perfect.
"Porter is very confident at the moment, but when he feels Kell's power, I think that it's going to be a different story and I can't see Porter lasting the 12 rounds."
Brook is undefeated in 32 professional encounters, while Porter has 24 wins and one draw from his 25 matches in the paid ranks.