British light-heavyweight Joshua Buatsi secured a victory yet underwhelming debut to life under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's punch output seemingly secured him a judges' verdict in a competitive 10-round contest at the Manchester venue.
But the challenger found success, landing some telling clean blows in a match where the styles failed to mesh, with constant grabbing spoiling the contest.
Two judges judged it 96-94 in favor to Buatsi, with the remaining official a even score.
"It wasn't my best showing but what defines champions? They prevail," Buatsi said in the ring as he was met by disapproval from the audience.
The Londoner returned to winning ways after his initial pro loss - an memorable fight with his previous opponent in last winter - to claim a twentieth professional win.
Various analysts believed Parker, 31, deserved the decision but the contender experienced the second defeat of his 28-fight career.
The Champion Secures Triumph in Uneventful Match
Buatsi's progress has plateaued in the past few years through promotional disputes and considerable time of inactivity.
Earlier forecast as the next star of UK boxing, he now stands trying to realize his potential - perhaps in what appears to be a last chance - under recent support.
He began cautiously, with both competitors willing to be aggressive in a calculated opening few rounds.
The challenger, whose sole prior loss came at the lower weight class against his former opponent in 2022 after damaging his hand mid-fight, was making just his second fight at light heavyweight.
By the halfway stage, Parker had made it competitive, causing problems for Buatsi, who worked behind a tight guard to narrowly win the more effective rounds.
The spectators' acceptance diminished amid frequent holding, with Parker falling to the floor various instances during close exchanges.
The underdog finished the stronger, but multiple periods were difficult to score. Nevertheless, previous titleholder the boxing analyst - providing analysis for Dazn - had it strongly leaning for the challenger, giving Buatsi just only one stanza.
The victor advances, but with the presence of hard hitter Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the road to world glory - on this evidence - still looks steep.
Ricky Hatton Homage Inspires Manchester Derby Contest
In the secondary fight, Arthur beat Rea by split-decision in a outstanding all-Manchester battle to capture the continental championship.
The arena became quiet for a deeply poignant memorial prior to the bout as ten bells sounded in memory of the city's native Ricky Hatton, who succumbed aged 46 in September.
It was the first major televised boxing event in the location since Hatton's passing. As the concluding sound disappeared, the stillness transitioned to a roar of "only one Hatton".
The competition during the bout equaled the feeling. Arthur was sent to the canvas by a significant blow directly to the chin in the second round, standing uncertainly to his position on the referee's count.
But the Manchester United-red equipment of Arthur soon came forward again. Across the ring, Rea - in opposing blue - was marked up early, heavy swelling developing under his orbital area as the fighters exchanged hard punches.
The challenger, twenty-seven, delivered effective strikes but Arthur, 34, fought fire with fire with stinging uppercuts as the action hardly diminished over the full distance.
The victor narrowly won the decision with scoring tallies of 114-114, 115-113 and clearer margin, but each competitor were relentless and full of heart, everything their idol Hatton represented.
