UK light-heavyweight Joshua Buatsi earned a win yet disappointing start to life under fresh promotional backing with a decision win over Parker in the Manchester arena.
The Olympic medal winner's work rate likely gained him a majority decision in a competitive ten-round bout at the Co-op Live Arena.
But Parker landed significant shots, delivering effective single shots in a match where the styles failed to mesh, with excessive holding disrupting the action.
Two judges scored it 96-94 to the winner, with the final judge a 95-95.
"It wasn't my best display but what defines champions? They succeed," the victor said in the ring as he was faced boos from the spectators.
The British fighter got back to winning after his maiden career setback - an epic battle with his previous opponent in February - to earn a 20th career victory.
Various analysts believed Parker, 31, merited the victory but the contender suffered the second defeat of his 28-fight career.
The Champion Claims Win in Uneventful Match
Buatsi's progress has stalled in recent times through contract problems and extended periods of inactivity.
Earlier forecast as the rising talent of UK boxing, he now finds himself trying to achieve expectations - possibly in what feels like a last chance - under new promotional backing.
He started carefully, with the two boxers ready to be aggressive in a tentative initial rounds.
The challenger, whose single earlier setback came at super-middleweight against his former opponent in two years ago after suffering a hand injury mid-fight, was making just his second fight at the higher weight class.
By the halfway stage, the underdog had kept it close, causing problems for the favorite, who operated with a protective stance to narrowly win the more effective rounds.
The crowd's patience wore thin because of regular grabbing, with the challenger falling to the floor various instances during grappling.
The underdog ended stronger, but multiple periods were hard to judge. However, ex-champion the boxing analyst - working with the broadcast network - had it particularly wide for Parker, giving Buatsi just only one stanza.
Buatsi moves on, but with the presence of power puncher David Benavidez, and Eastern European fighter Bivol dominating the weight class, the path to championship success - based on this performance - still seems difficult.
Hatton Tribute Motivates All-Manchester Contest
In the supporting bout, Arthur overcame Bradley Rea by close verdict in a outstanding Manchester vs Manchester fight to win the European light-heavyweight belt.
The arena became quiet for a profoundly moving memorial ahead of the contest as ten bells sounded in honor of the city's native Hatton, who succumbed at age 46 in last month.
It was the first major televised fight night in the urban center since the passing of Hatton. As the last bell disappeared, the quiet gave way to a chant of "there's only one Ricky Hatton".
The action within the ring equaled the feeling. The eventual winner was knocked down by a left hook directly to the face in the early round, getting up unsteadily to his feet on the count of eight.
But the Manchester United-red attire of the fighter soon came forward again. Across the ring, the opponent - in competitive blue - was injured soon, significant bruising appearing under his eye as the duo exchanged punishing blows.
The challenger, 27, delivered effective strikes but Arthur, thirty-four, matched the intensity with powerful upward punches as the action rarely dropped over 12 rounds.
Arthur barely secured the result with official scores of draw, 115-113 and wider gap, but both fighters were persistent and brave, everything their idol the legend represented.
