The Doncaster man, boxing 20 miles downstream in Sheffield as chief support to Kell Brook, was finding the going tough against Darwin Zamora, a late call-up following the withdrawal of Vusi Malinga, until the Nicaraguan opted not to get off his stool at the end of the eighth round.
With open cuts on the left-hand side of his face, the relative unknown decided against soaking up further punishment, conceding the riches on offer for winning this final eliminator.
And while there was no doubt he was headed for a defeat on the cards when he called time, McDonnell will have been relieved not to have had to do another four rounds with him, for everything he was throwing seemed to be making little impression, other than the facial wound which developed from the sixth round onwards.
"The pressure was massive out there, but I knew what I could do, I know I'm the best in the world and now I have to prove it," said McDonnell afterwards.
"I thought I had beat him, but he kept coming back."
Zamora had not been coming at all until Monday when McDonnell, who is now 20-2-1, thought he was resigned to a regulation 12-rounder.
The IBF sanctioned tonight to be the eliminator Malinga was meant for, though, and McDonnell practically ran to the ring fuelled by a maelstrom of South Yorkshire pride, with a number of his own fans joining those already waiting for Brook.
He jumped out of his corner too, working behind a ranging left-hand jab and getting through with an early right, as well as two quick lefts.
He soaked up an early rib hit from Zamora and then made progress through the middle of his defences, shielding himself from danger under a low, hunched guard.
He upped the ante in the second round, a three-shot combination making an impression, although he was unable to fully unload after pinning Zamora on the ropes with a sweet left-hander.
McDonnell changed things up a little in the third, connecting from close range to try and combat Zamora's compact style, but took aim from distance again in the next, before he visibly winced twice as Zamora finally opened up with a straight shot from each hand.
A couple of heavy rights got McDonnell back on top in the fifth, although he missed what would have been a finishing shot, going over the top with a left-hander after rocking Zamora back with a quick one-two.
The meatier connection off his strongest side continued into the sixth and Zamora did well to skip away from trouble after taking two fizzing shots on the ropes, and bounced back with a handy left-right of his own after drinking up a five-strong flurry from McDonnell in the corner - a trick he repeated in the seventh.
Things were fast becoming attritional, with Zamora showing little signs of wear short of a the cut above his left eye, and even when McDonnell finally forced him to sway back on his heels with a slick right, he found him quickly coming back for more.
That ended at the end of the eighth, though, giving McDonnell the moment he had wanted.