British light heavyweight Joshua Buatsi earned a win yet underwhelming beginning to his career under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's activity level appeared to earn him a judges' verdict in a close ten-round fight at the boxing arena.
But the challenger landed significant shots, connecting with powerful clean blows in a contest where the techniques failed to mesh, with excessive holding disrupting the fight.
Two judges judged it 96-94 to Buatsi, with the remaining official a even score.
"It wasn't my greatest display but how do champions respond? They win," the victor commented in the ring as he was met by negative reactions from the spectators.
The Londoner returned to winning ways after his maiden career setback - an thrilling contest with his previous opponent in last winter - to secure a 20th pro success.
Some ringside observers believed Parker, 31, merited the decision but the contender endured the second defeat of his 28-match professional journey.
Buatsi Secures Win in Dull Encounter
His career development has plateaued in the past few years through management issues and long spells of inactivity.
Earlier forecast as the next star of UK boxing, he now finds himself trying to realize his potential - maybe in what appears to be a last chance - under fresh management.
He began cautiously, with the two boxers ready to take risks in a tentative initial rounds.
Parker, whose sole prior loss came at super-middleweight against John Ryder in 2022 after breaking his hand in the contest, was making just his second bout at the higher weight class.
By the halfway stage, the underdog had kept it close, creating difficulties for the favorite, who operated with a solid defense to edge the more effective rounds.
The audience's tolerance wore thin because of regular grabbing, with Parker falling to the floor various instances during grappling.
Parker finished the stronger, but many rounds were difficult to score. However, ex-champion the boxing analyst - providing analysis for the broadcast network - had it strongly leaning for Parker, awarding Buatsi just only one stanza.
The victor advances, but with the presence of hard hitter Benavidez, and Russian star Dmitry Bivol leading the category, the journey to titles - based on this performance - still looks steep.
Ricky Hatton Homage Motivates Manchester Derby Contest
In the supporting bout, Lyndon Arthur overcame Rea by close verdict in a excellent local rivalry fight to capture the European light-heavyweight championship.
The stadium grew hushed for a deeply poignant memorial ahead of the contest as a bell ceremony tolled in memory of the city's native Hatton, who succumbed at age 46 in September.
It was the first major televised fight night in the urban center since Hatton's death. As the final chime disappeared, the silence transitioned to a shout of "only one Hatton".
The action inside the ropes matched the feeling. The eventual winner was sent to the canvas by a left hook directly to the face in the second round, rising shakily to his footing on the eight count.
But the Manchester United-red shorts of the fighter soon came forward again. Across the ring, Rea - in competitive blue - was injured quickly, noticeable damage appearing under his eye as the fighters traded hard punches.
The challenger, twenty-seven, delivered effective strikes but Arthur, 34, fought fire with fire with powerful upward punches as the intensity seldom decreased over twelve rounds.
Arthur edged the decision with official scores of even score, 115-113 and wider gap, but the two athletes were persistent and full of heart, all the qualities their inspiration the legend represented.
