British light-heavyweight Joshua Buatsi secured a victory yet disappointing start to life under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's punch output likely gained him a majority decision in a competitive 10-round contest at the boxing arena.
But the challenger landed significant shots, connecting with powerful individual punches in a fight where the techniques didn't complement each other, with constant grabbing spoiling the contest.
The officials had it 96-94 for to Buatsi, with the final judge a 95-95.
"It wasn't my greatest performance but what defines champions? They succeed," the victor commented in the ring as he was greeted with boos from the crowd.
The British fighter returned to winning ways after his initial pro loss - an thrilling contest with Callum Smith in February - to secure a 20th pro success.
Various analysts felt Parker, 31, deserved the victory but the British boxer suffered the second setback of his 28-match professional journey.
The Champion Earns Victory in Slow Affair
Buatsi's progress has slowed in recent years through contract problems and considerable time of lack of fights.
Once tipped as the next star of UK boxing, he now is positioned trying to fulfil that early promise - possibly in what seems like a last chance - under new promotional backing.
He opened tentatively, with neither fighter willing to overcommit in a cagey early stages.
The challenger, whose only previous defeat came at super-middleweight against John Ryder in 2022 after damaging his hand mid-fight, was competing in only his second appearance at light-heavyweight.
By the middle rounds, the underdog had made it competitive, making life awkward for the favorite, who worked behind a protective stance to narrowly win the better rounds.
The audience's tolerance diminished because of frequent holding, with the challenger slipping to the canvas multiple occasions during clinches.
Parker finished the stronger, but multiple periods were difficult to score. Nevertheless, previous titleholder Carl Frampton - commentating on Dazn - had it strongly leaning for Parker, giving Buatsi just one round.
Buatsi moves on, but with the likes of power puncher David Benavidez, and Russian star Bivol leading the category, the path to championship success - judging by this showing - still seems difficult.
Hatton Memorial Inspires All-Manchester Battle
In the co-main event, Arthur defeated Bradley Rea by close verdict in a excellent all-Manchester fight to secure the European light-heavyweight belt.
The venue grew hushed for a extremely emotional memorial before the fight as 10 bells sounded in honor of Manchester's own Hatton, who succumbed at age 46 in September.
It was the premier televised match card in the city since Hatton's passing. As the last bell ended, the silence became a shout of "only one Ricky Hatton".
The competition during the bout matched the sentiment. The eventual winner was floored by a powerful punch flush on the jaw in the initial stage, rising shakily to his position on the count of eight.
But the red-colored equipment of Arthur soon pressed ahead again. Across the ring, the opponent - in opposing blue - was marked up early, noticeable damage appearing under his orbital area as the pair landed punishing blows.
Rea, 27, landed power punches but Arthur, thirty-four, matched the intensity with stinging uppercuts as the intensity seldom decreased over the full distance.
Arthur narrowly won the result with judges scorecards of even score, 115-113 and wider gap, but each competitor were relentless and full of heart, exactly what their idol the legend represented.
