UK light heavyweight Joshua Buatsi made a winning yet underwhelming start to his career under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's activity level seemingly secured him a majority decision in a tight 10-round fight at the Manchester venue.
But Parker landed significant shots, landing some telling individual punches in a match where the techniques failed to mesh, with frequent clinching spoiling the fight.
The officials scored it 96-94 in favor to Buatsi, with the final judge a 95-95.
"It wasn't my greatest display but what defines champions? They win," the victor said in the ring as he was met by negative reactions from the audience.
The Londoner returned to winning ways after his initial pro loss - an epic battle with his previous opponent in February - to secure a 20th pro success.
Several boxing experts thought the challenger, thirty-one, merited the victory but the British boxer experienced the second setback of his 28-fight career.
The Champion Earns Triumph in Dull Affair
Buatsi's progress has slowed in recent years through contract problems and considerable time of inactivity.
Once tipped as the rising talent of UK boxing, he now finds himself trying to fulfil that early promise - possibly in what feels like a final roll of the dice - under fresh management.
He began cautiously, with the two boxers willing to overcommit in a calculated opening few rounds.
Parker, whose sole prior loss came at super-middleweight against John Ryder in 2022 after breaking his hand in the contest, was participating in his second appearance at the higher weight class.
By the fight's midpoint, Parker had stayed in contention, making life awkward for Buatsi, who utilized a tight guard to barely take the more effective stanzas.
The audience's tolerance wore thin during constant clinching, with Parker falling to the floor various instances during close exchanges.
The underdog ended stronger, but many rounds were hard to judge. However, previous titleholder Carl Frampton - working with Dazn - had it particularly wide for the challenger, awarding Buatsi just one round.
Buatsi progresses, but with the likes of power puncher David Benavidez, and Eastern European fighter Bivol dominating the weight class, the path to championship success - based on this performance - still appears challenging.
Ricky Hatton Memorial Inspires Manchester Derby Fight
In the secondary fight, Arthur overcame Bradley Rea by split-decision in a excellent all-Manchester fight to win the European light-heavyweight championship.
The stadium became quiet for a deeply poignant tribute before the fight as a bell ceremony rang in memory of local hero Ricky Hatton, who passed away aged 46 in last month.
It was the initial significant broadcast match card in the urban center since the passing of Hatton. As the concluding sound faded, the quiet gave way to a chant of "only one Hatton".
The competition inside the ropes matched the sentiment. Arthur was sent to the canvas by a left hook squarely on the face in the early round, standing uncertainly to his feet on the count of eight.
But the traditional red shorts of the fighter soon came forward again. On the opposite side, the opponent - in opposing blue - was marked up soon, noticeable damage developing under his orbital area as the duo landed hard punches.
Rea, twenty-seven, landed power punches but Arthur, 34, responded in kind with powerful upward punches as the intensity seldom decreased over 12 rounds.
The victor edged the verdict with official scores of even score, 115-113 and clearer margin, but the two athletes were persistent and full of heart, all the qualities their idol the legend represented.
