British light-heavyweight Joshua Buatsi secured a victory yet disappointing beginning to life under recent management with a unanimous decision over Parker in Manchester.
The Olympic medal winner's punch output appeared to earn him a judges' verdict in a tight 10-round fight at the boxing arena.
But Parker had his moments, delivering effective clean blows in a match where the approaches didn't complement each other, with frequent clinching marring the fight.
Two judges judged it 96-94 to Buatsi, with the remaining official a 95-95.
"It wasn't my greatest display but what do champions do? They win," Buatsi commented in the arena as he was greeted with boos from the audience.
The British fighter returned to winning ways after his initial pro loss - an thrilling contest with his previous opponent in last winter - to secure a 20th pro success.
Several boxing experts felt Parker, 31, merited the win but the contender endured the second defeat of his 28-fight career.
The Champion Claims Victory in Dull Affair
His career development has stalled in the past few years through promotional disputes and long spells of lack of fights.
Previously predicted as the next star of the British fight scene, he now is positioned trying to fulfil that early promise - maybe in what seems like a last chance - under recent support.
He began cautiously, with both competitors ready to take risks in a cagey early stages.
Parker, whose only previous defeat came at the lower weight class against John Ryder in two years ago after damaging his hand during the bout, was making just his second bout at the higher weight class.
By the middle rounds, the underdog had stayed in contention, creating difficulties for the favorite, who operated with a solid defense to barely take the more effective stanzas.
The spectators' acceptance decreased during frequent holding, with the challenger slipping to the canvas various instances during close exchanges.
The underdog closed well, but several stanzas were hard to judge. Nevertheless, ex-champion the boxing analyst - providing analysis for Dazn - had it particularly wide for the challenger, giving Buatsi just one round.
The victor moves on, but with the likes of hard hitter David Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the path to championship success - on this evidence - still appears challenging.
Ricky Hatton Tribute Inspires All-Manchester Fight
In the secondary fight, Arthur overcame Bradley Rea by split-decision in a terrific local rivalry fight to capture the European light-heavyweight belt.
The arena fell silent for a profoundly moving tribute prior to the bout as a bell ceremony tolled in memory of the city's native Hatton, who succumbed aged 46 in recently.
It was the initial significant broadcast fight night in the location since the passing of Hatton. As the final chime disappeared, the quiet transitioned to a chant of "only one Ricky Hatton".
The fighting during the bout reflected the sentiment. The eventual winner was floored by a significant blow directly to the chin in the early round, getting up unsteadily to his feet on the referee's count.
But the traditional red attire of the fighter soon advanced again. Across the ring, the opponent - in opposing blue - was marked up early, noticeable damage developing under his face as the pair traded punishing blows.
Rea, twenty-seven, connected with strong blows but the champion, 34, fought fire with fire with effective uppercuts as the intensity hardly diminished over 12 rounds.
Arthur barely secured the verdict with scoring tallies of even score, close margin and clearer margin, but the two athletes were relentless and brave, exactly what their hero the legend embodied.
