British light-heavyweight Joshua Buatsi earned a win yet unimpressive beginning to his career under recent management with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's punch output appeared to earn him a majority decision in a close ten-round contest at the boxing arena.
But the challenger found success, delivering effective individual punches in a match where the styles never quite clicked, with excessive holding spoiling the action.
Two judges judged it 96-94 to Buatsi, with the third a even score.
"It wasn't my finest showing but what do champions do? They succeed," the victor commented in the ring as he was faced negative reactions from the audience.
The Londoner bounced back after his first professional defeat - an memorable fight with Callum Smith in February - to secure a twentieth professional win.
Various analysts felt Parker, thirty-one, should have been awarded the decision but the British boxer suffered the second setback of his 28-bout career.
The Champion Claims Win in Slow Affair
Buatsi's progress has plateaued in the past few years through contract problems and long spells of inactivity.
Once tipped as the rising talent of British boxing, he now is positioned trying to fulfil that early promise - perhaps in what appears to be a final roll of the dice - under recent support.
He opened tentatively, with the two boxers willing to be aggressive in a calculated initial rounds.
Parker, whose single earlier setback came at super middleweight against John Ryder in two years ago after breaking his hand in the contest, was competing in only his second appearance at the higher weight class.
By the halfway stage, Parker had made it competitive, making life awkward for Buatsi, who worked behind a tight guard to edge the better rounds.
The crowd's patience wore thin because of frequent holding, with Parker slipping to the canvas multiple occasions during close exchanges.
The underdog closed well, but many rounds were hard to judge. Nevertheless, former world champion the boxing analyst - working with Dazn - had it strongly leaning for Parker, giving Buatsi just one round.
Buatsi moves on, but with the likes of hard hitter Benavidez, and Russian star Bivol dominating the weight class, the journey to titles - based on this performance - still seems difficult.
Ricky Hatton Homage Motivates All-Manchester Contest
In the secondary fight, Arthur overcame Bradley Rea by close verdict in a outstanding all-Manchester fight to win the European light heavyweight championship.
The stadium fell silent for a profoundly moving memorial prior to the bout as a bell ceremony rang in memory of the city's native Hatton, who passed away aged 46 in recently.
It was the initial significant broadcast fight night in the urban center since Hatton's passing. As the final chime ended, the silence became a shout of "there's only one Hatton".
The fighting inside the ropes equaled the emotion. The eventual winner was sent to the canvas by a significant blow directly to the jaw in the initial stage, getting up unsteadily to his footing on the referee's count.
But the Manchester United-red attire of Arthur soon came forward again. Across the ring, Rea - in opposing blue - was marked up early, significant bruising developing under his face as the duo traded powerful shots.
Rea, 27, connected with strong blows but the champion, thirty-four, matched the intensity with effective uppercuts as the action hardly diminished over the full distance.
The victor edged the verdict with scoring tallies of draw, 115-113 and wider gap, but each competitor were determined and brave, exactly what their hero the legend represented.
