UK light heavyweight Joshua Buatsi made a winning yet unimpressive debut to his career under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic medal winner's activity level likely gained him a judges' verdict in a tight 10-round contest at the Manchester venue.
But the challenger landed significant shots, connecting with powerful individual punches in a contest where the styles didn't complement each other, with frequent clinching marring the action.
The scoring judges had it 96-94 to the winner, with the remaining official a 95-95.
"It wasn't my greatest display but what defines champions? They succeed," Buatsi stated in the arena as he was met by boos from the crowd.
The British fighter returned to winning ways after his maiden career setback - an epic battle with his previous opponent in last winter - to secure a 20th pro success.
Various analysts believed Parker, 31, should have been awarded the decision but the contender experienced the second defeat of his 28-fight career.
The Champion Claims Win in Slow Encounter
Buatsi's progress has plateaued in the past few years through promotional disputes and extended periods of lack of fights.
Once tipped as the next star of British boxing, he now is positioned trying to realize his potential - perhaps in what feels like a last chance - under recent support.
He opened tentatively, with neither fighter prepared to be aggressive in a calculated opening few rounds.
Parker, whose sole prior loss came at the lower weight class against John Ryder in 2022 after breaking his hand in the contest, was competing in only his second appearance at the higher weight class.
By the middle rounds, Parker had made it competitive, causing problems for Buatsi, who worked behind a tight guard to edge the cleaner rounds.
The crowd's patience diminished during regular grabbing, with the challenger falling to the floor multiple occasions during close exchanges.
Parker ended stronger, but many rounds were challenging to assess. However, former world champion the boxing analyst - working with the broadcast network - had it particularly wide for Parker, awarding Buatsi just only one stanza.
The victor progresses, but with the caliber of hard hitter David Benavidez, and Russian champion Bivol leading the category, the road to world glory - on this evidence - still appears challenging.
Ricky Hatton Homage Inspires Manchester Derby Fight
In the supporting bout, Arthur defeated Rea by divided decision in a outstanding all-Manchester contest to secure the continental championship.
The stadium grew hushed for a deeply poignant memorial before the fight as ten bells rang in remembrance of local hero Ricky Hatton, who succumbed at age 46 in recently.
It was the first major televised fight night in the location since Hatton's death. As the concluding sound faded, the quiet gave way to a shout of "there's just one Ricky Hatton".
The competition during the bout matched the sentiment. Arthur was sent to the canvas by a left hook flush on the chin in the early round, rising shakily to his position on the count of eight.
But the red-colored shorts of Arthur soon pressed ahead again. Across the ring, Rea - in rival City blue - was injured soon, noticeable damage appearing under his eye as the duo traded punishing blows.
The challenger, twenty-seven, connected with strong blows but Arthur, thirty-four, matched the intensity with stinging uppercuts as the intensity seldom decreased over twelve rounds.
The victor barely secured the decision with judges scorecards of draw, close margin and clearer margin, but each competitor were determined and full of heart, all the qualities their inspiration Hatton stood for.
