British light heavyweight Joshua Buatsi secured a victory yet unimpressive start to his career under new promoters with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output seemingly secured him a judges' verdict in a close ten-round contest at the Co-op Live Arena.
But the challenger landed significant shots, connecting with powerful single shots in a match where the approaches never quite clicked, with frequent clinching marring the action.
The officials scored it 96-94 for to Buatsi, with the third a 95-95 draw.
"It wasn't my greatest performance but what defines champions? They win," the victor said in the arena as he was met by negative reactions from the audience.
The Londoner got back to winning after his initial pro loss - an epic battle with his previous opponent in last winter - to earn a 20th pro success.
Some ringside observers felt Parker, 31, should have been awarded the decision but the contender experienced the second setback of his 28-fight career.
Buatsi Secures Victory in Slow Match
His career development has slowed in recent times through management issues and long spells of inactivity.
Previously predicted as the rising talent of UK boxing, he now is positioned trying to realize his potential - perhaps in what feels like a final roll of the dice - under recent support.
He began cautiously, with the two boxers prepared to overcommit in a cagey initial rounds.
The challenger, whose only previous defeat came at super-middleweight against John Ryder in 2022 after suffering a hand injury mid-fight, was participating in his second fight at the higher weight class.
By the fight's midpoint, the underdog had made it competitive, creating difficulties for Buatsi, who worked behind a protective stance to narrowly win the better periods.
The audience's tolerance wore thin amid regular grabbing, with Parker falling to the floor several times during clinches.
The underdog closed well, but multiple periods were difficult to score. Nevertheless, ex-champion Carl Frampton - commentating on the broadcast network - had it particularly wide for the challenger, awarding Buatsi just a single round.
The victor moves on, but with the likes of heavy hitter Benavidez, and Russian star Dmitry Bivol leading the category, the journey to titles - judging by this showing - still looks steep.
Hatton Memorial Fuels All-Manchester Contest
In the co-main event, Lyndon Arthur defeated Rea by close verdict in a terrific Manchester vs Manchester battle to capture the European light-heavyweight championship.
The venue became quiet for a extremely emotional tribute before the fight as ten bells rang in memory of local hero Hatton, who passed away at age 46 in recently.
It was the first major televised boxing event in the city since the passing of Hatton. As the last bell disappeared, the quiet became a shout of "only one Hatton".
The action inside the ropes reflected the emotion. The eventual winner was floored by a left hook flush on the jaw in the early round, rising shakily to his footing on the count of eight.
But the traditional red shorts of the fighter soon advanced again. On the opposite side, the opponent - in rival City blue - was injured quickly, significant bruising appearing under his face as the pair exchanged punishing blows.
Rea, 27, delivered effective strikes but Arthur, thirty-four, fought fire with fire with powerful upward punches as the pace seldom decreased over twelve rounds.
Arthur edged the decision with judges scorecards of even score, 115-113 and clearer margin, but both fighters were persistent and full of heart, everything their inspiration the legend represented.
