David Haye delayed telling Dereck Chisora about his change of opponent to stop the heavyweight downing tools.
Chisora was due to face former world champion Joseph Parker in London on Saturday as he looked to continue his resurgence.
But the New Zealander pulled out after suffering a spider bite and was replaced by David Price.
But Haye, who manages Chisora, kept the news from his fighter until the alternative opponent was secured.
"When you're geared up to fight someone who's well respected, who's a former world champion, and someone who holds a win over Andy Ruiz Jr who is the unified champion, you know you've got to mentally gear up," he told Mirror Fighting's After The Bell.
"As soon as I heard of the spider bite I left it for a while before I told him because there were a few hard sessions I wanted to get through with Joseph Parker in mind.
"I didn't want to tell him he wasn't fighting Parker without knowing who he was fighting. If there was ever a time to take his foot off the pedal it would have been then.
"I spoke to Eddie Hearn about potential opponents and David Price’s name kept coming up. He’s on a winning streak and he looked great against Dave Allen.
"It was an obvious fight; he brings a lot of danger, he’s 6 foot 9 and I've sparred with him many times.
"I believed at one stage he could be heavyweight champion of the world. It didn't work out that way but he’s got a second wave of his career and looked as good as ever in his last fight."
Chisora is the odds-on favourite to beat Price with bookies Betfair and while Haye predicts a potentially rocky start, he expects his man to come through.
"Dereck has sparred with David before so he's felt his power," he added. "It's hard to get past that long jab; he has a piston-like jab.
"They don't look hard on TV but I've eaten a few of them. Hes a heavy-handed man and to beat him you've got to come through a few problems.
"Dereck knows he might have to weather an early storm to keep the educated pressure on and grind him down.
"It's going to be very, very explosive early but I see Dereck's physicality being the key to smothering David Price."
