British light heavyweight Joshua Buatsi earned a win yet unimpressive start to life under new promoters with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's activity level seemingly secured him a split decision in a close 10-round fight at the boxing arena.
But Parker found success, connecting with powerful individual punches in a contest where the styles never quite clicked, with excessive holding spoiling the contest.
The scoring judges had it 96-94 in favor to the winner, with the remaining official a 95-95.
"It wasn't my best display but what defines champions? They prevail," the victor commented in the ringside as he was faced negative reactions from the crowd.
The British fighter got back to winning after his maiden career setback - an memorable fight with his previous opponent in February - to claim a 20th pro success.
Some ringside observers believed the challenger, 31, merited the win but the Derbyshire fighter endured the second setback of his 28-match career.
The Champion Earns Win in Dull Encounter
Buatsi's progress has stalled in recent years through contract problems and extended periods of inactivity.
Previously predicted as the future of UK boxing, he now stands trying to fulfil that early promise - perhaps in what seems like a last chance - under new promotional backing.
He started carefully, with the two boxers prepared to overcommit in a calculated opening few rounds.
Parker, whose only previous defeat came at super-middleweight against John Ryder in 2022 after suffering a hand injury during the bout, was making just his second bout at the higher weight class.
By the fight's midpoint, Parker had stayed in contention, making life awkward for Buatsi, who utilized a solid defense to edge the more effective rounds.
The audience's tolerance wore thin during frequent holding, with the challenger going down several times during close exchanges.
The underdog closed well, but several stanzas were difficult to score. Nevertheless, ex-champion the boxing analyst - working with Dazn - had it particularly wide for Parker, awarding Buatsi just a single round.
Buatsi moves on, but with the presence of power puncher David Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the journey to titles - on this evidence - still looks steep.
Hatton Memorial Fuels Local Rivalry Fight
In the secondary fight, Arthur overcame Bradley Rea by divided decision in a outstanding all-Manchester contest to capture the continental championship.
The venue became quiet for a deeply poignant tribute before the fight as 10 bells tolled in remembrance of Manchester's own Hatton, who succumbed aged 46 in recently.
It was the premier televised match card in the urban center since Hatton's passing. As the concluding sound faded, the quiet transitioned to a roar of "only one Hatton".
The competition inside the ropes equaled the emotion. Arthur was knocked down by a left hook flush on the jaw in the initial stage, getting up unsteadily to his position on the referee's count.
But the Manchester United-red equipment of Arthur soon came forward again. Across the ring, Rea - in opposing blue - was injured soon, significant bruising forming under his face as the duo exchanged powerful shots.
Rea, 27, delivered effective strikes but the champion, thirty-four, fought fire with fire with stinging uppercuts as the action hardly diminished over 12 rounds.
The victor narrowly won the verdict with judges scorecards of draw, close margin and wider gap, but both fighters were relentless and courageous, everything their hero Hatton embodied.
