UK light heavyweight Joshua Buatsi secured a victory yet disappointing start to life under recent management with a points victory over Parker in Manchester.
The Olympic bronze medallist's activity level seemingly secured him a split decision in a close ten-round fight at the boxing arena.
But the challenger found success, connecting with powerful single shots in a fight where the styles didn't complement each other, with frequent clinching marring the action.
The officials judged it 96-94 in favor to Buatsi, with the final judge a even score.
"It wasn't my best display but how do champions respond? They win," the victor stated in the arena as he was met by negative reactions from the audience.
The British fighter bounced back after his first professional defeat - an thrilling contest with his previous opponent in last winter - to claim a twentieth professional win.
Various analysts thought Parker, thirty-one, should have been awarded the decision but the contender experienced the second loss of his 28-fight professional journey.
The Champion Secures Win in Uneventful Match
The fighter's advancement has plateaued in recent times through contract problems and extended periods of inactivity.
Earlier forecast as the next star of the British fight scene, he now finds himself trying to fulfil that early promise - possibly in what feels like a last chance - under fresh management.
He opened tentatively, with neither fighter willing to be aggressive in a tentative opening few rounds.
Parker, whose single earlier setback came at the lower weight class against John Ryder in two years ago after damaging his hand mid-fight, was making just his second fight at light heavyweight.
By the halfway stage, the underdog had made it competitive, causing problems for Buatsi, who utilized a solid defense to barely take the better stanzas.
The crowd's patience diminished during constant clinching, with the challenger going down various instances during close exchanges.
The underdog finished the stronger, but multiple periods were hard to judge. Although, previous titleholder the boxing analyst - commentating on Dazn - had it significantly in favor for the challenger, giving Buatsi just only one stanza.
Buatsi advances, but with the presence of heavy hitter David Benavidez, and Russian champion Bivol leading the category, the road to world glory - judging by this showing - still seems difficult.
Ricky Hatton Tribute Inspires All-Manchester Contest
In the secondary fight, Arthur overcame Bradley Rea by divided decision in a outstanding all-Manchester contest to secure the continental title.
The arena grew hushed for a deeply poignant homage prior to the bout as 10 bells sounded in memory of Manchester's own Ricky Hatton, who passed away 46 years old in September.
It was the premier televised fight night in the urban center since Hatton's passing. As the concluding sound disappeared, the quiet became a roar of "there's only one Ricky Hatton".
The competition inside the ropes matched the feeling. The eventual winner was floored by a powerful punch flush on the chin in the early round, getting up unsteadily to his feet on the referee's count.
But the traditional red equipment of Arthur soon advanced again. Across the ring, Rea - in rival City blue - was injured soon, significant bruising forming under his eye as the fighters landed powerful shots.
Rea, 27, landed power punches but the champion, 34, fought fire with fire with effective uppercuts as the action seldom decreased over the full distance.
Arthur barely secured the decision with judges scorecards of even score, narrow lead and clearer margin, but each competitor were persistent and brave, all the qualities their idol Hatton represented.
