British light-heavyweight Joshua Buatsi earned a win yet disappointing debut to life under new promoters with a unanimous decision over Parker in Manchester.
The Olympic medal winner's work rate seemingly secured him a majority decision in a close ten-round bout at the Manchester venue.
But the challenger landed significant shots, delivering effective clean blows in a fight where the styles failed to mesh, with excessive holding disrupting the fight.
The officials had it 96-94 in favor to the winner, with the final judge a 95-95.
"It wasn't my finest showing but what defines champions? They prevail," the victor said in the ring as he was faced negative reactions from the audience.
The Londoner got back to winning after his initial pro loss - an epic battle with Callum Smith in February - to earn a 20th career victory.
Some ringside observers thought the challenger, thirty-one, should have been awarded the victory but the British boxer endured the second loss of his 28-fight career.
The Champion Claims Win in Slow Encounter
Buatsi's progress has plateaued in recent times through contract problems and long spells of inactivity.
Once tipped as the rising talent of the British fight scene, he now stands trying to fulfil that early promise - perhaps in what feels like a final roll of the dice - under new promotional backing.
He started carefully, with the two boxers willing to be aggressive in a cagey early stages.
Parker, whose single earlier setback came at super-middleweight against John Ryder in two years ago after damaging his hand in the contest, was competing in only his second bout at the higher weight class.
By the halfway stage, the underdog had made it competitive, creating difficulties for the favorite, who utilized a protective stance to barely take the better rounds.
The crowd's patience wore thin because of regular grabbing, with the challenger going down various instances during grappling.
Parker finished the stronger, but multiple periods were hard to judge. Although, ex-champion the boxing analyst - working with Dazn - had it strongly leaning for the challenger, awarding Buatsi just a single round.
The victor advances, but with the likes of power puncher Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the path to championship success - on this evidence - still appears challenging.
Hatton Tribute Inspires Manchester Derby Fight
In the co-main event, Arthur overcame Bradley Rea by divided decision in a outstanding all-Manchester battle to secure the European light-heavyweight belt.
The arena fell silent for a extremely emotional tribute before the fight as a bell ceremony tolled in remembrance of Manchester's own Hatton, who passed away 46 years old in September.
It was the first major televised boxing event in the city since Hatton's passing. As the last bell ended, the quiet gave way to a shout of "there's just one Ricky Hatton".
The competition within the ring equaled the feeling. The eventual winner was floored by a powerful punch squarely on the jaw in the second round, getting up unsteadily to his footing on the eight count.
But the traditional red attire of Arthur soon pressed ahead again. On the opposite side, the opponent - in rival City blue - was marked up soon, heavy swelling developing under his face as the duo landed powerful shots.
Rea, 27, landed power punches but Arthur, 34, matched the intensity with powerful upward punches as the action rarely dropped over twelve rounds.
The victor barely secured the verdict with scoring tallies of draw, narrow lead and 115-112, but the two athletes were determined and courageous, all the qualities their hero Hatton embodied.
