UK light-heavyweight Joshua Buatsi earned a win yet underwhelming beginning to his career under recent management with a decision win over Parker in Manchester.
The Olympic bronze medallist's work rate appeared to earn him a split decision in a competitive 10-round bout at the boxing arena.
But Parker had his moments, delivering effective single shots in a match where the styles failed to mesh, with excessive holding marring the fight.
The officials had it 96-94 in favor to the winner, with the remaining official a 95-95.
"It wasn't my greatest showing but how do champions respond? They succeed," Buatsi stated in the ring as he was greeted with boos from the spectators.
The British fighter returned to winning ways after his maiden career setback - an epic battle with Callum Smith in February - to secure a 20th career victory.
Various analysts felt the challenger, 31, merited the decision but the British boxer suffered the second loss of his 28-match professional journey.
The Champion Claims Victory in Dull Match
The fighter's advancement has plateaued in the past few years through promotional disputes and extended periods of inactivity.
Previously predicted as the next star of the British fight scene, he now finds himself trying to achieve expectations - possibly in what feels like a final roll of the dice - under fresh management.
He began cautiously, with neither fighter willing to be aggressive in a tentative opening few rounds.
Parker, whose single earlier setback came at super middleweight against his former opponent in two years ago after suffering a hand injury in the contest, was participating in his second fight at light heavyweight.
By the middle rounds, Parker had made it competitive, causing problems for Buatsi, who operated with a tight guard to barely take the more effective periods.
The crowd's patience decreased because of frequent holding, with the challenger going down various instances during clinches.
The underdog closed well, but many rounds were difficult to score. Although, previous titleholder Carl Frampton - commentating on Dazn - had it strongly leaning for Parker, awarding Buatsi just a single round.
Buatsi advances, but with the presence of hard hitter David Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the path to championship success - based on this performance - still looks steep.
Ricky Hatton Homage Fuels All-Manchester Fight
In the co-main event, Arthur defeated Rea by divided decision in a terrific all-Manchester fight to secure the continental belt.
The venue became quiet for a deeply poignant tribute ahead of the contest as 10 bells rang in honor of the city's native Ricky Hatton, who died 46 years old in September.
It was the premier televised match card in the city since Hatton's passing. As the concluding sound ended, the silence became a roar of "there's only one Ricky Hatton".
The action inside the ropes equaled the sentiment. The eventual winner was sent to the canvas by a left hook flush on the chin in the initial stage, rising shakily to his position on the eight count.
But the Manchester United-red equipment of the fighter soon came forward again. On the opposite side, the opponent - in rival City blue - was injured early, noticeable damage forming under his orbital area as the pair landed powerful shots.
The challenger, twenty-seven, connected with strong blows but Arthur, 34, matched the intensity with stinging uppercuts as the pace rarely dropped over 12 rounds.
The victor barely secured the verdict with judges scorecards of even score, 115-113 and clearer margin, but both fighters were determined and full of heart, everything their hero the legend embodied.
