The Belfast man retained his title on a split decision after 12 magnificent rounds, but the punishment he took from the Mexican's crisp and clean punches throughout the 12 rounds must raise doubts as to the wisdom of his continuing to squeeze a featherweight's body into the bantamweight division.
There was considerable unease among the 4,000 crowd as they waited for the verdict and had it gone Bueno's way McCullough would not have had serious grounds for complaint. The Irishman finished the fight with his face a mass of bruises and cuts, testament to the accuracy of the challenger's punches, and at times it was only his indomitable will to win that kept him going. "He's the bravest fighter I've ever seen,'' said the WBO super- middleweight champion, Steve Collins, who knows about such matters.
McCullough's trainer, Eddie Futch, who has seen all the great champions of the past 60 years and trained his share of them, is fond of comparing McCullough with the immortal Henry Armstrong, whose non-stop style made him the only man in history to hold world titles in three divisions simultaneously. On last night's evidence, McCullough matches Armstrong on work-rate but not on technique.
He never looked like subduing the classy and spirited challenger, himself a former WBC champion at super-flyweight, and was clearly taken aback by the level of resistance he encountered from a man he had expected to crush in fairly short order. "I'm going to need a good rest after this," he acknowledged.
There were no knock-downs, despite the volume of punches landed by both men, but if the fight were scored in terms of visible damage inflicted, McCullough came a poor second. He bled from the nose and left ear, and by the ninth his face was so badly swollen that there were fears his jaw may have been broken.
Bueno took his share of punishment too, fighting from the early rounds with a swelling under the left eye, but he never lost his composure despite the relentless pressure.
When the verdict was announced, Bueno sank to his knees weeping in disappointment, and the relief in McCullough's corner told its own story. The damage McCullough sustained must cast doubt on the $500,000 date he had booked with Luigi Camputaro in Las Vegas on 7 June. The prospect of him facing WBO featherweight champion Naseem Hamed must now seem remote. Had Naseem landed those rights last night instead of the light-hitting Bueno, the damage to McCullough's career might have been irreversible.