British light-heavyweight Joshua Buatsi secured a victory yet unimpressive beginning to his career under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's punch output appeared to earn him a judges' verdict in a competitive 10-round fight at the Manchester venue.
But the challenger found success, landing some telling individual punches in a fight where the techniques failed to mesh, with frequent clinching marring the contest.
The scoring judges had it 96-94 for to the winner, with the final judge a 95-95 draw.
"It wasn't my greatest display but what defines champions? They win," Buatsi commented in the ring as he was faced negative reactions from the crowd.
The Londoner returned to winning ways after his initial pro loss - an thrilling contest with his previous opponent in last winter - to secure a twentieth professional win.
Some ringside observers believed Parker, 31, should have been awarded the win but the contender endured the second setback of his 28-bout career.
Buatsi Earns Triumph in Uneventful Match
Buatsi's progress has slowed in the past few years through promotional disputes and long spells of lack of fights.
Previously predicted as the next star of British boxing, he now is positioned trying to achieve expectations - maybe in what seems like a last chance - under recent support.
He opened tentatively, with both competitors prepared to be aggressive in a cagey initial rounds.
Parker, whose sole prior loss came at super-middleweight against his former opponent in 2022 after breaking his hand mid-fight, was participating in his second appearance at the higher weight class.
By the middle rounds, the underdog had kept it close, making life awkward for Buatsi, who utilized a tight guard to edge the more effective stanzas.
The audience's tolerance decreased because of frequent holding, with Parker slipping to the canvas various instances during clinches.
The underdog closed well, but many rounds were hard to judge. However, former world champion Carl Frampton - commentating on Dazn - had it significantly in favor for the challenger, awarding Buatsi just only one stanza.
Buatsi progresses, but with the caliber of heavy hitter David Benavidez, and Russian star Bivol leading the category, the path to championship success - based on this performance - still appears challenging.
Ricky Hatton Tribute Fuels Manchester Derby Contest
In the secondary fight, Arthur overcame Bradley Rea by divided decision in a outstanding local rivalry battle to secure the continental title.
The stadium fell silent for a extremely emotional homage before the fight as 10 bells sounded in remembrance of the city's native Hatton, who passed away at age 46 in last month.
It was the first major televised fight night in the urban center since Hatton's death. As the concluding sound ended, the stillness transitioned to a chant of "there's just one Hatton".
The action within the ring equaled the emotion. Arthur was floored by a powerful punch flush on the jaw in the early round, standing uncertainly to his position on the eight count.
But the Manchester United-red shorts of Arthur soon pressed ahead again. From the other corner, the opponent - in opposing blue - was damaged early, significant bruising developing under his eye as the duo traded punishing blows.
Rea, 27, landed power punches but the champion, 34, matched the intensity with powerful upward punches as the pace seldom decreased over 12 rounds.
Arthur narrowly won the result with judges scorecards of 114-114, narrow lead and wider gap, but the two athletes were determined and courageous, all the qualities their hero Hatton stood for.
