UK light heavyweight Joshua Buatsi earned a win yet disappointing start to his career under recent management with a decision win over Parker in the Manchester arena.
The Olympic medal winner's punch output seemingly secured him a majority decision in a tight ten-round fight at the boxing arena.
But Parker had his moments, landing some telling single shots in a fight where the approaches never quite clicked, with constant grabbing marring the fight.
Two judges scored it 96-94 to the winner, with the final judge a 95-95.
"It wasn't my greatest display but what defines champions? They prevail," the victor said in the ringside as he was faced negative reactions from the crowd.
The Londoner got back to winning after his initial pro loss - an epic battle with Callum Smith in last winter - to earn a 20th pro success.
Various analysts felt the challenger, thirty-one, merited the victory but the British boxer experienced the second defeat of his 28-match career.
The Champion Earns Win in Slow Encounter
The fighter's advancement has plateaued in the past few years through contract problems and considerable time of inactivity.
Previously predicted as the future of the British fight scene, he now finds himself trying to fulfil that early promise - perhaps in what feels like a last chance - under fresh management.
He began cautiously, with the two boxers willing to be aggressive in a cagey initial rounds.
The challenger, whose sole prior loss came at super middleweight against his former opponent in 2022 after breaking his hand during the bout, was participating in his second appearance at the higher weight class.
By the fight's midpoint, the underdog had stayed in contention, causing problems for Buatsi, who utilized a tight guard to barely take the more effective periods.
The crowd's patience diminished amid constant clinching, with Parker going down various instances during clinches.
Parker ended stronger, but multiple periods were hard to judge. Although, ex-champion the boxing analyst - working with Dazn - had it particularly wide for the challenger, giving Buatsi just one round.
The victor advances, but with the likes of heavy hitter Benavidez, and Russian star Bivol dominating the weight class, the road to world glory - based on this performance - still looks steep.
Hatton Homage Motivates Manchester Derby Battle
In the co-main event, Arthur beat Rea by divided decision in a terrific local rivalry contest to secure the European light-heavyweight title.
The arena grew hushed for a profoundly moving tribute ahead of the contest as 10 bells rang in remembrance of local hero Hatton, who died aged 46 in September.
It was the premier televised fight night in the city since Hatton's death. As the last bell faded, the stillness transitioned to a shout of "only one Ricky Hatton".
The competition inside the ropes reflected the sentiment. The eventual winner was sent to the canvas by a significant blow squarely on the chin in the second round, rising shakily to his position on the eight count.
But the red-colored equipment of Arthur soon pressed ahead again. On the opposite side, Rea - in opposing blue - was damaged early, heavy swelling appearing under his face as the fighters landed hard punches.
Rea, twenty-seven, connected with strong blows but the champion, thirty-four, fought fire with fire with powerful upward punches as the intensity seldom decreased over the full distance.
The victor barely secured the verdict with judges scorecards of 114-114, 115-113 and clearer margin, but each competitor were determined and brave, everything their inspiration Hatton represented.
