British light heavyweight Joshua Buatsi made a winning yet underwhelming beginning to life under recent management with a points victory over Parker in the Manchester arena.
The Olympic medal winner's work rate likely gained him a majority decision in a competitive 10-round bout at the Manchester venue.
But the challenger had his moments, connecting with powerful single shots in a contest where the approaches failed to mesh, with excessive holding spoiling the contest.
Two judges judged it 96-94 in favor to the winner, with the third a even score.
"It wasn't my greatest performance but what do champions do? They win," Buatsi said in the ringside as he was faced boos from the crowd.
The Londoner bounced back after his maiden career setback - an epic battle with Callum Smith in February - to claim a twentieth professional win.
Several boxing experts felt Parker, 31, merited the decision but the contender suffered the second defeat of his 28-match professional journey.
Buatsi Secures Win in Uneventful Match
The fighter's advancement has slowed in recent times through contract problems and considerable time of inactivity.
Previously predicted as the rising talent of British boxing, he now stands trying to realize his potential - perhaps in what feels like a ultimate opportunity - under recent support.
He started carefully, with neither fighter prepared to take risks in a calculated initial rounds.
Parker, whose only previous defeat came at the lower weight class against his former opponent in 2022 after damaging his hand during the bout, was making just his second appearance at light-heavyweight.
By the middle rounds, Parker had kept it close, making life awkward for Buatsi, who operated with a solid defense to barely take the better periods.
The spectators' acceptance wore thin during frequent holding, with the challenger going down several times during clinches.
Parker finished the stronger, but several stanzas were challenging to assess. Although, former world champion the boxing analyst - commentating on Dazn - had it particularly wide for Parker, giving Buatsi just one round.
The victor progresses, but with the presence of heavy hitter Benavidez, and Russian champion Dmitry Bivol leading the category, the path to championship success - on this evidence - still appears challenging.
Hatton Tribute Motivates Local Rivalry Battle
In the supporting bout, Lyndon Arthur defeated Rea by split-decision in a outstanding all-Manchester battle to secure the European light heavyweight championship.
The stadium grew hushed for a extremely emotional memorial ahead of the contest as ten bells sounded in remembrance of Manchester's own Hatton, who succumbed 46 years old in last month.
It was the first major televised match card in the city since the passing of Hatton. As the last bell ended, the stillness became a chant of "only one Hatton".
The action inside the ropes equaled the feeling. The eventual winner was sent to the canvas by a powerful punch flush on the face in the second round, rising shakily to his position on the eight count.
But the Manchester United-red attire of Arthur soon advanced again. From the other corner, the opponent - in rival City blue - was damaged quickly, heavy swelling appearing under his eye as the pair exchanged hard punches.
Rea, twenty-seven, landed power punches but the champion, thirty-four, fought fire with fire with stinging uppercuts as the action hardly diminished over 12 rounds.
Arthur narrowly won the result with official scores of even score, narrow lead and clearer margin, but both fighters were relentless and courageous, exactly what their inspiration Hatton stood for.
