British light-heavyweight Joshua Buatsi earned a win yet unimpressive debut to life under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's work rate likely gained him a judges' verdict in a close 10-round bout at the Co-op Live Arena.
But the challenger landed significant shots, delivering effective single shots in a fight where the styles didn't complement each other, with frequent clinching spoiling the action.
Two judges judged it 96-94 to the winner, with the final judge a 95-95.
"It wasn't my best showing but what defines champions? They win," Buatsi commented in the ring as he was greeted with boos from the audience.
The British fighter returned to winning ways after his initial pro loss - an thrilling contest with his previous opponent in February - to earn a 20th pro success.
Various analysts thought the challenger, thirty-one, deserved the win but the contender suffered the second defeat of his 28-fight career.
The Champion Claims Victory in Dull Encounter
The fighter's advancement has stalled in recent times through management issues and extended periods of lack of fights.
Previously predicted as the next star of British boxing, he now is positioned trying to fulfil that early promise - perhaps in what seems like a ultimate opportunity - under new promotional backing.
He opened tentatively, with the two boxers ready to overcommit in a cagey initial rounds.
Parker, whose only previous defeat came at super middleweight against John Ryder in 2022 after damaging his hand mid-fight, was participating in his second bout at the higher weight class.
By the middle rounds, Parker had kept it close, creating difficulties for the favorite, who worked behind a tight guard to barely take the more effective rounds.
The spectators' acceptance wore thin because of constant clinching, with the challenger going down multiple occasions during clinches.
The underdog ended stronger, but several stanzas were difficult to score. However, ex-champion the boxing analyst - working with the broadcast network - had it strongly leaning for the challenger, scoring for Buatsi just only one stanza.
The victor progresses, but with the likes of power puncher Benavidez, and Russian champion Dmitry Bivol ruling the division, the path to championship success - judging by this showing - still seems difficult.
Hatton Homage Inspires Local Rivalry Contest
In the secondary fight, Arthur overcame Bradley Rea by split-decision in a terrific local rivalry battle to secure the European light heavyweight championship.
The venue grew hushed for a extremely emotional memorial before the fight as 10 bells tolled in honor of local hero Ricky Hatton, who passed away 46 years old in recently.
It was the premier televised fight night in the city since Hatton's death. As the last bell faded, the quiet gave way to a shout of "there's only one Ricky Hatton".
The action inside the ropes matched the emotion. The eventual winner was floored by a significant blow flush on the face in the second round, standing uncertainly to his position on the count of eight.
But the traditional red attire of the fighter soon advanced again. On the opposite side, the opponent - in opposing blue - was injured early, heavy swelling developing under his face as the duo landed punishing blows.
The challenger, twenty-seven, landed power punches but the champion, 34, fought fire with fire with effective uppercuts as the action hardly diminished over twelve rounds.
The victor edged the decision with judges scorecards of even score, narrow lead and clearer margin, but both fighters were persistent and full of heart, everything their idol the legend represented.
