UK light-heavyweight Joshua Buatsi made a winning yet unimpressive beginning to his career under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's activity level appeared to earn him a majority decision in a close 10-round bout at the Manchester venue.
But Parker found success, delivering effective single shots in a match where the techniques failed to mesh, with excessive holding marring the fight.
The officials scored it 96-94 for to Buatsi, with the third a even score.
"It wasn't my greatest showing but what defines champions? They prevail," Buatsi stated in the arena as he was faced boos from the spectators.
The British fighter bounced back after his first professional defeat - an epic battle with his previous opponent in February - to claim a 20th career victory.
Some ringside observers believed Parker, 31, deserved the victory but the British boxer endured the second loss of his 28-bout professional journey.
Buatsi Claims Win in Dull Encounter
Buatsi's progress has plateaued in recent times through management issues and extended periods of inactivity.
Once tipped as the next star of UK boxing, he now finds himself trying to achieve expectations - maybe in what feels like a last chance - under recent support.
He began cautiously, with the two boxers prepared to overcommit in a calculated early stages.
Parker, whose single earlier setback came at super middleweight against his former opponent in two years ago after suffering a hand injury in the contest, was participating in his second fight at the higher weight class.
By the middle rounds, Parker had stayed in contention, making life awkward for Buatsi, who worked behind a protective stance to narrowly win the more effective stanzas.
The audience's tolerance wore thin amid frequent holding, with Parker slipping to the canvas several times during clinches.
Parker ended stronger, but multiple periods were challenging to assess. However, ex-champion the boxing analyst - working with the broadcast network - had it significantly in favor for the challenger, giving Buatsi just a single round.
Buatsi advances, but with the presence of hard hitter David Benavidez, and Eastern European fighter Bivol ruling the division, the journey to titles - on this evidence - still seems difficult.
Ricky Hatton Tribute Fuels Manchester Derby Battle
In the co-main event, Arthur defeated Bradley Rea by split-decision in a excellent all-Manchester battle to secure the continental title.
The venue fell silent for a profoundly moving homage before the fight as 10 bells rang in memory of local hero Hatton, who passed away at age 46 in September.
It was the premier televised match card in the location since Hatton's passing. As the final chime faded, the quiet gave way to a chant of "there's only one Ricky Hatton".
The competition within the ring matched the sentiment. The eventual winner was knocked down by a left hook flush on the face in the initial stage, standing uncertainly to his position on the referee's count.
But the Manchester United-red attire of the fighter soon advanced again. Across the ring, Rea - in opposing blue - was damaged early, noticeable damage developing under his face as the fighters traded powerful shots.
Rea, twenty-seven, landed power punches but the champion, thirty-four, responded in kind with powerful upward punches as the intensity hardly diminished over the full distance.
The victor barely secured the verdict with scoring tallies of draw, close margin and 115-112, but each competitor were determined and courageous, all the qualities their hero the legend embodied.
