UK light-heavyweight Joshua Buatsi secured a victory yet underwhelming beginning to life under recent management with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's work rate appeared to earn him a majority decision in a competitive 10-round contest at the boxing arena.
But Parker had his moments, landing some telling clean blows in a contest where the approaches never quite clicked, with frequent clinching disrupting the action.
Two judges judged it 96-94 in favor to Buatsi, with the third a 95-95.
"It wasn't my greatest display but what do champions do? They win," Buatsi stated in the arena as he was greeted with boos from the spectators.
The British fighter returned to winning ways after his maiden career setback - an epic battle with Callum Smith in last winter - to earn a twentieth professional win.
Some ringside observers felt the challenger, 31, deserved the victory but the British boxer suffered the second defeat of his 28-bout professional journey.
The Champion Earns Triumph in Slow Match
Buatsi's progress has slowed in recent years through promotional disputes and extended periods of inactivity.
Earlier forecast as the rising talent of UK boxing, he now finds himself trying to achieve expectations - perhaps in what feels like a ultimate opportunity - under fresh management.
He began cautiously, with neither fighter willing to overcommit in a tentative opening few rounds.
Parker, 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 appearance at light heavyweight.
By the halfway stage, Parker had kept it close, creating difficulties for the favorite, who operated with a tight guard to edge the better rounds.
The crowd's patience wore thin amid constant clinching, with the challenger going down various instances during grappling.
Parker closed well, but many rounds were difficult to score. Although, previous titleholder the boxing analyst - commentating on Dazn - had it particularly wide for Parker, scoring for Buatsi just a single round.
Buatsi advances, but with the caliber of hard hitter David Benavidez, and Russian champion Bivol ruling the division, the road to world glory - on this evidence - still appears challenging.
Ricky Hatton Tribute Motivates Manchester Derby Battle
In the supporting bout, Arthur defeated Rea by close verdict in a excellent all-Manchester battle to secure the continental championship.
The stadium fell silent for a deeply poignant tribute before the fight as 10 bells rang in honor of local hero Ricky Hatton, who succumbed 46 years old in September.
It was the premier televised match card in the city since Hatton's death. As the concluding sound disappeared, the silence gave way to a chant of "only one Ricky Hatton".
The fighting during the bout matched the emotion. Arthur was floored by a left hook directly to the face in the early round, standing uncertainly to his footing on the count of eight.
But the red-colored shorts of the fighter soon pressed ahead again. On the opposite side, Rea - in competitive blue - was damaged early, heavy swelling forming under his eye as the duo exchanged hard punches.
Rea, 27, connected with strong blows but the champion, 34, fought fire with fire with stinging uppercuts as the pace seldom decreased over the full distance.
The victor edged the verdict with judges scorecards of 114-114, close margin and 115-112, but the two athletes were persistent and courageous, all the qualities their inspiration the legend represented.
