UK light-heavyweight Joshua Buatsi secured a victory yet disappointing debut to life under recent management with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's work rate appeared to earn him a judges' verdict in a tight ten-round contest at the Manchester venue.
But the challenger found success, landing some telling single shots in a match where the styles never quite clicked, with excessive holding disrupting the action.
The officials had it 96-94 in favor to Buatsi, with the third a even score.
"It wasn't my best showing but what do champions do? They win," Buatsi said in the ringside as he was met by boos from the spectators.
The Londoner got back to winning after his maiden career setback - an memorable fight with his previous opponent in last winter - to earn a twentieth professional win.
Several boxing experts believed Parker, thirty-one, deserved the victory but the contender suffered the second loss of his 28-fight career.
The Champion Claims Triumph in Slow Match
The fighter's advancement has stalled in recent years through contract problems and long spells of inactivity.
Previously predicted as the rising talent of UK boxing, he now is positioned trying to achieve expectations - perhaps in what appears to be a final roll of the dice - under recent support.
He opened tentatively, with neither fighter prepared to be aggressive in a calculated initial rounds.
The challenger, whose single earlier setback came at the lower weight class against his former opponent in two years ago after suffering a hand injury mid-fight, was participating in his second appearance at the higher weight class.
By the halfway stage, the underdog had stayed in contention, creating difficulties for Buatsi, who worked behind a solid defense to barely take the better periods.
The spectators' acceptance diminished because of regular grabbing, with Parker slipping to the canvas various instances during grappling.
The underdog ended stronger, but many rounds were hard to judge. However, previous titleholder Carl Frampton - providing analysis for the broadcast network - had it particularly wide for Parker, awarding Buatsi just a single round.
The victor moves on, but with the presence of power puncher Benavidez, and Russian star Dmitry Bivol leading the category, the path to championship success - judging by this showing - still seems difficult.
Ricky Hatton Memorial Fuels Manchester Derby Contest
In the supporting bout, Arthur beat Bradley Rea by close verdict in a terrific all-Manchester contest to capture the European light heavyweight championship.
The venue fell silent for a profoundly moving homage prior to the bout as a bell ceremony sounded in memory of Manchester's own Ricky Hatton, who succumbed aged 46 in last month.
It was the first major televised boxing event in the location since the passing of Hatton. As the final chime disappeared, the quiet became a chant of "there's only one Hatton".
The competition during the bout reflected the sentiment. The eventual winner was sent to the canvas by a powerful punch directly to the chin in the early round, standing uncertainly to his footing on the count of eight.
But the traditional red attire of Arthur soon pressed ahead again. Across the ring, the opponent - in opposing blue - was injured soon, significant bruising forming under his orbital area as the pair traded punishing blows.
Rea, twenty-seven, delivered effective strikes but Arthur, thirty-four, responded in kind with effective uppercuts as the intensity seldom decreased over twelve rounds.
The victor narrowly won the verdict with official scores of 114-114, 115-113 and wider gap, but the two athletes were relentless and full of heart, exactly what their idol Hatton represented.
