British light-heavyweight Joshua Buatsi made a winning yet disappointing start to life under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's work rate appeared to earn him a judges' verdict in a close ten-round fight at the Manchester venue.
But the challenger landed significant shots, delivering effective individual punches in a match where the approaches never quite clicked, with frequent clinching spoiling the fight.
The scoring judges scored it 96-94 to the winner, with the remaining official a even score.
"It wasn't my greatest display but how do champions respond? They win," the victor stated in the ring as he was faced disapproval from the crowd.
The British fighter returned to winning ways after his initial pro loss - an memorable fight with his previous opponent in February - to earn a twentieth professional win.
Some ringside observers felt Parker, thirty-one, merited the win but the British boxer suffered the second loss of his 28-bout career.
Buatsi Earns Triumph in Uneventful Match
Buatsi's progress has stalled in the past few years through promotional disputes and extended periods of inactivity.
Earlier forecast as the next star of British boxing, he now is positioned trying to achieve expectations - possibly in what appears to be a last chance - under new promotional backing.
He started carefully, with the two boxers prepared to be aggressive in a cagey initial rounds.
The challenger, whose single earlier setback came at super middleweight against his former opponent in 2022 after breaking his hand during the bout, was making just his second fight at light heavyweight.
By the middle rounds, Parker had stayed in contention, making life awkward for Buatsi, who worked behind a solid defense to edge the cleaner stanzas.
The crowd's patience diminished amid frequent holding, with Parker falling to the floor various instances during clinches.
Parker finished the stronger, but several stanzas were hard to judge. However, previous titleholder the boxing analyst - providing analysis for the broadcast network - had it significantly in favor for the challenger, awarding Buatsi just only one stanza.
The victor moves on, but with the caliber of power puncher David Benavidez, and Eastern European fighter Bivol ruling the division, the path to championship success - judging by this showing - still seems difficult.
Ricky Hatton Memorial Inspires All-Manchester Fight
In the supporting bout, Arthur beat Rea by split-decision in a excellent local rivalry fight to secure the European light heavyweight title.
The venue fell silent for a profoundly moving homage before the fight as 10 bells sounded in remembrance of local hero Hatton, who passed away at age 46 in last month.
It was the initial significant broadcast fight night in the urban center since the passing of Hatton. As the last bell disappeared, the quiet gave way to a roar of "there's only one Ricky Hatton".
The action during the bout equaled the sentiment. Arthur was floored by a left hook squarely on the chin in the second round, standing uncertainly to his position on the count of eight.
But the Manchester United-red equipment of the fighter soon pressed ahead again. On the opposite side, Rea - in rival City blue - was damaged soon, noticeable damage appearing under his orbital area as the duo exchanged punishing blows.
Rea, twenty-seven, delivered effective strikes but the champion, thirty-four, matched the intensity with powerful upward punches as the pace hardly diminished over twelve rounds.
The victor edged the decision with judges scorecards of draw, 115-113 and 115-112, but the two athletes were persistent and courageous, all the qualities their idol the legend embodied.
