UK light-heavyweight Joshua Buatsi earned a win yet unimpressive beginning to life under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's work rate seemingly secured him a majority decision in a tight 10-round bout at the Manchester venue.
But Parker landed significant shots, connecting with powerful clean blows in a contest where the approaches didn't complement each other, with constant grabbing spoiling the contest.
The officials had it 96-94 to the winner, with the remaining official a 95-95.
"It wasn't my finest showing but what do champions do? They prevail," Buatsi said in the arena as he was greeted with boos from the spectators.
The British fighter returned to winning ways after his maiden career setback - an thrilling contest with Callum Smith in last winter - to secure a 20th career victory.
Some ringside observers felt the challenger, 31, should have been awarded the decision but the contender suffered the second loss of his 28-bout professional journey.
The Champion Earns Victory in Slow Affair
His career development has stalled in recent years through management issues and long spells of lack of fights.
Earlier forecast as the next star of the British fight scene, he now stands trying to fulfil that early promise - perhaps in what seems like a last chance - under new promotional backing.
He began cautiously, with neither fighter willing to be aggressive in a tentative initial rounds.
The challenger, whose single earlier setback came at super middleweight against his former opponent in two years ago after breaking his hand in the contest, was participating in his second bout at light heavyweight.
By the fight's midpoint, Parker had made it competitive, creating difficulties for the favorite, who utilized a solid defense to edge the more effective rounds.
The spectators' acceptance diminished because of frequent holding, with Parker going down various instances during clinches.
Parker ended stronger, but multiple periods were difficult to score. Although, previous titleholder Carl Frampton - working with the broadcast network - had it strongly leaning for Parker, scoring for Buatsi just only one stanza.
The victor progresses, but with the caliber of power puncher David Benavidez, and Russian star Dmitry Bivol leading the category, the road to world glory - on this evidence - still looks steep.
Ricky Hatton Tribute Motivates All-Manchester Contest
In the supporting bout, Arthur overcame Rea by split-decision in a outstanding local rivalry fight to win the European light-heavyweight championship.
The venue became quiet for a profoundly moving tribute prior to the bout as a bell ceremony rang in memory of the city's native Ricky Hatton, who succumbed 46 years old in last month.
It was the premier televised match card in the city since Hatton's death. As the concluding sound faded, the quiet became a chant of "there's only one Hatton".
The fighting during the bout matched the sentiment. The eventual winner was floored by a significant blow directly to the chin in the early round, standing uncertainly to his footing on the eight count.
But the traditional red attire of the fighter soon came forward again. Across the ring, the opponent - in rival City blue - was damaged soon, heavy swelling appearing under his eye as the duo landed hard punches.
The challenger, twenty-seven, connected with strong blows but the champion, 34, matched the intensity with powerful upward punches as the pace seldom decreased over 12 rounds.
The victor narrowly won the verdict with official scores of even score, close margin and wider gap, but the two athletes were relentless and full of heart, everything their hero Hatton embodied.
