British light-heavyweight Joshua Buatsi earned a win yet disappointing start to life under recent management with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's work rate likely gained him a judges' verdict in a close 10-round bout at the Manchester venue.
But Parker landed significant shots, landing some telling single shots in a match where the approaches never quite clicked, with excessive holding marring the contest.
The officials had it 96-94 to the winner, with the remaining official a even score.
"It wasn't my best display but what do champions do? They succeed," the victor stated in the arena as he was faced disapproval from the crowd.
The Londoner returned to winning ways after his maiden career setback - an memorable fight with his previous opponent in February - to claim a twentieth professional win.
Various analysts thought the challenger, thirty-one, merited the decision but the contender suffered the second setback of his 28-fight professional journey.
Buatsi Secures Victory in Slow Encounter
His career development has slowed in recent times through contract problems and considerable time of lack of fights.
Once tipped as the future of UK boxing, he now is positioned trying to fulfil that early promise - perhaps in what seems like a last chance - under new promotional backing.
He began cautiously, with neither fighter prepared to take risks in a cagey early stages.
The challenger, whose sole prior loss came at the lower weight class against his former opponent in 2022 after suffering a hand injury during the bout, was competing in only his second fight at light-heavyweight.
By the halfway stage, Parker had kept it close, making life awkward for Buatsi, who worked behind a protective stance to edge the more effective periods.
The crowd's patience decreased during regular grabbing, with the challenger slipping to the canvas various instances during grappling.
The underdog closed well, but multiple periods were hard to judge. Although, ex-champion the boxing analyst - providing analysis for Dazn - had it significantly in favor for Parker, giving Buatsi just only one stanza.
The victor advances, but with the likes of heavy hitter Benavidez, and Russian star Dmitry Bivol dominating the weight class, the road to world glory - judging by this showing - still seems difficult.
Ricky Hatton Tribute Fuels Manchester Derby Battle
In the co-main event, Lyndon Arthur beat Rea by split-decision in a terrific local rivalry battle to win the European light-heavyweight belt.
The venue became quiet for a profoundly moving memorial before the fight as 10 bells sounded in memory of local hero Hatton, who succumbed aged 46 in recently.
It was the first major televised match card in the location since Hatton's passing. As the final chime ended, the quiet became a chant of "only one Ricky Hatton".
The action within the ring reflected the emotion. Arthur was floored by a significant blow squarely on the jaw in the early round, standing uncertainly to his feet on the eight count.
But the traditional red equipment of the fighter soon came forward again. Across the ring, Rea - in opposing blue - was marked up soon, significant bruising appearing under his orbital area as the pair landed punishing blows.
The challenger, twenty-seven, landed power punches but Arthur, thirty-four, matched the intensity with stinging uppercuts as the action rarely dropped over the full distance.
The victor barely secured the decision with official scores of draw, close margin and clearer margin, but each competitor were persistent and full of heart, all the qualities their inspiration the legend represented.
