UK light heavyweight Joshua Buatsi earned a win yet disappointing debut to his career under fresh promotional backing with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's work rate appeared to earn him a split decision in a tight 10-round bout at the Manchester venue.
But Parker had his moments, delivering effective clean blows in a contest where the techniques never quite clicked, with constant grabbing marring the contest.
Two judges judged it 96-94 for to Buatsi, with the remaining official a even score.
"It wasn't my finest display but what do champions do? They win," Buatsi commented in the arena as he was greeted with negative reactions from the audience.
The British fighter bounced back after his initial pro loss - an memorable fight with his previous opponent in last winter - to claim a 20th pro success.
Various analysts felt Parker, 31, should have been awarded the win but the contender endured the second loss of his 28-fight professional journey.
The Champion Earns Victory in Slow Affair
His career development has slowed in recent years through promotional disputes and extended periods of lack of fights.
Earlier forecast as the next star of British boxing, he now stands trying to achieve expectations - maybe in what appears to be a final roll of the dice - under recent support.
He started carefully, with neither fighter ready to overcommit in a tentative initial rounds.
The challenger, whose only previous defeat came at super middleweight against his former opponent in 2022 after suffering a hand injury mid-fight, was participating in his second appearance at the higher weight class.
By the halfway stage, the underdog had kept it close, creating difficulties for Buatsi, who worked behind a protective stance to edge the cleaner stanzas.
The crowd's patience wore thin amid frequent holding, with Parker going down various instances during clinches.
Parker ended stronger, but multiple periods were difficult to score. Nevertheless, former world champion the boxing analyst - working with Dazn - had it strongly leaning for Parker, awarding Buatsi just one round.
Buatsi advances, but with the likes of power puncher Benavidez, and Russian star Dmitry Bivol dominating the weight class, the journey to titles - based on this performance - still looks steep.
Hatton Homage Inspires Local Rivalry Fight
In the co-main event, Arthur beat Rea by divided decision in a outstanding local rivalry fight to win the continental championship.
The arena became quiet for a extremely emotional tribute ahead of the contest as ten bells sounded in honor of Manchester's own Hatton, who succumbed at age 46 in September.
It was the premier televised fight night in the urban center since Hatton's passing. As the concluding sound faded, the silence became a chant of "there's only one Ricky Hatton".
The competition during the bout reflected the emotion. Arthur was knocked down by a left hook flush on the face in the initial stage, getting up unsteadily to his position on the referee's count.
But the Manchester United-red shorts of the fighter soon advanced again. On the opposite side, the opponent - in rival City blue - was marked up quickly, significant bruising appearing under his eye as the duo traded hard punches.
Rea, twenty-seven, connected with strong blows but the champion, 34, matched the intensity with stinging uppercuts as the pace seldom decreased over the full distance.
The victor barely secured the decision with official scores of 114-114, close margin and 115-112, but both fighters were determined and full of heart, everything their hero Hatton embodied.
