British light heavyweight Joshua Buatsi earned a win yet unimpressive debut to his career under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's activity level likely gained him a split decision in a close ten-round fight at the Co-op Live Arena.
But Parker found success, delivering effective individual punches in a match where the styles didn't complement each other, with constant grabbing disrupting the action.
The officials scored it 96-94 to the winner, with the final judge a 95-95 draw.
"It wasn't my finest showing but what do champions do? They succeed," Buatsi said in the ring as he was greeted with boos from the crowd.
The British fighter returned to winning ways after his maiden career setback - an thrilling contest with his previous opponent in February - to secure a 20th pro success.
Various analysts believed Parker, thirty-one, should have been awarded the win but the British boxer suffered the second loss of his 28-bout career.
The Champion Earns Triumph in Dull Affair
The fighter's advancement has slowed in recent years through contract problems and extended periods of inactivity.
Earlier forecast as the rising talent of British boxing, he now finds himself trying to fulfil that early promise - possibly in what appears to be a last chance - under fresh management.
He started carefully, with the two boxers ready to overcommit in a tentative early stages.
Parker, whose sole prior loss came at super middleweight against John Ryder in 2022 after damaging his hand in the contest, was participating in his second fight at the higher weight class.
By the fight's midpoint, the underdog had stayed in contention, making life awkward for the favorite, who worked behind a tight guard to barely take the more effective stanzas.
The spectators' acceptance wore thin during constant clinching, with Parker going down several times during close exchanges.
The underdog closed well, but several stanzas were challenging to assess. However, previous titleholder the boxing analyst - commentating on Dazn - had it particularly wide for the challenger, giving Buatsi just a single round.
The victor progresses, but with the caliber of heavy hitter David Benavidez, and Russian star Dmitry Bivol leading the category, the road to world glory - on this evidence - still seems difficult.
Ricky Hatton Homage Inspires Manchester Derby Fight
In the secondary fight, Arthur beat Bradley Rea by close verdict in a terrific Manchester vs Manchester contest to secure the European light heavyweight belt.
The stadium fell silent for a deeply poignant tribute ahead of the contest as 10 bells sounded in remembrance of local hero Hatton, who passed away at age 46 in recently.
It was the initial significant broadcast boxing event in the urban center since the passing of Hatton. As the concluding sound faded, the silence became a roar of "only one Ricky Hatton".
The action during the bout equaled the feeling. The eventual winner was floored by a significant blow squarely on the chin in the second round, standing uncertainly to his position on the eight count.
But the Manchester United-red attire of Arthur soon pressed ahead again. On the opposite side, Rea - in opposing blue - was injured quickly, noticeable damage developing under his face as the pair exchanged hard punches.
The challenger, twenty-seven, connected with strong blows but Arthur, 34, matched the intensity with powerful upward punches as the pace hardly diminished over 12 rounds.
The victor narrowly won the verdict with scoring tallies of draw, narrow lead and clearer margin, but the two athletes were relentless and brave, exactly what their hero Hatton represented.
