UK light heavyweight Joshua Buatsi made a winning yet underwhelming beginning to life under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's activity level appeared to earn him a split decision in a close 10-round fight at the boxing arena.
But Parker found success, connecting with powerful individual punches in a fight where the styles never quite clicked, with excessive holding spoiling the action.
The officials judged it 96-94 in favor to Buatsi, with the third a even score.
"It wasn't my best showing but what defines champions? They succeed," Buatsi said in the arena as he was met by disapproval from the crowd.
The Londoner bounced back after his initial pro loss - an memorable fight with Callum Smith in February - to claim a twentieth professional win.
Several boxing experts believed the challenger, thirty-one, should have been awarded the decision but the British boxer suffered the second loss of his 28-bout professional journey.
The Champion Claims Victory in Dull Affair
Buatsi's progress has plateaued in recent years through management issues and long spells of inactivity.
Earlier forecast as the rising talent of the British fight scene, he now stands trying to fulfil that early promise - maybe in what feels like a final roll of the dice - under fresh management.
He opened tentatively, with both competitors prepared to take risks in a cagey early stages.
Parker, whose only previous defeat came at super middleweight against his former opponent in 2022 after suffering a hand injury mid-fight, was competing in only his second appearance at light-heavyweight.
By the halfway stage, Parker had made it competitive, creating difficulties for the favorite, who utilized a solid defense to narrowly win the better stanzas.
The audience's tolerance decreased amid regular grabbing, with the challenger going down various instances during grappling.
The underdog closed well, but multiple periods were difficult to score. However, ex-champion the boxing analyst - working with the broadcast network - had it strongly leaning for the challenger, giving Buatsi just only one stanza.
Buatsi advances, but with the likes of heavy hitter Benavidez, and Russian star Dmitry Bivol leading the category, the path to championship success - based on this performance - still appears challenging.
Hatton Tribute Fuels All-Manchester Battle
In the co-main event, Arthur beat Rea by close verdict in a outstanding local rivalry battle to win the European light heavyweight championship.
The stadium grew hushed for a extremely emotional memorial before the fight as 10 bells sounded in memory of local hero Ricky Hatton, who passed away aged 46 in last month.
It was the first major televised match card in the urban center since Hatton's passing. As the concluding sound ended, the stillness transitioned to a shout of "there's just one Ricky Hatton".
The action during the bout equaled the feeling. The eventual winner was sent to the canvas by a powerful punch flush on the face in the early round, rising shakily to his feet on the eight count.
But the traditional red shorts of Arthur soon pressed ahead again. From the other corner, Rea - in opposing blue - was damaged quickly, significant bruising developing under his face as the fighters exchanged powerful shots.
The challenger, 27, connected with strong blows but the champion, 34, matched the intensity with stinging uppercuts as the pace rarely dropped over the full distance.
Arthur barely secured the result with judges scorecards of even score, close margin and wider gap, but the two athletes were relentless and courageous, all the qualities their inspiration the legend embodied.
