British light-heavyweight Joshua Buatsi earned a win yet disappointing start to his career under new promoters with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's work rate appeared to earn him a majority decision in a competitive ten-round bout at the Co-op Live Arena.
But the challenger had his moments, connecting with powerful single shots in a fight where the approaches didn't complement each other, with frequent clinching marring the contest.
Two judges scored it 96-94 to Buatsi, with the third a 95-95.
"It wasn't my best display but how do champions respond? They win," the victor stated in the arena as he was faced disapproval from the spectators.
The British fighter bounced back after his maiden career setback - an epic battle with his previous opponent in February - to earn a 20th pro success.
Some ringside observers believed Parker, thirty-one, should have been awarded the win but the British boxer suffered the second defeat of his 28-match professional journey.
Buatsi Secures Triumph in Dull Affair
His career development has slowed in the past few years through contract problems and long spells of inactivity.
Once tipped as the next star of the British fight scene, he now finds himself trying to fulfil that early promise - possibly in what appears to be a last chance - under recent support.
He opened tentatively, with neither fighter willing to take risks in a tentative early stages.
The challenger, whose single earlier setback came at the lower weight class against his former opponent in two years ago after damaging his hand in the contest, was competing in only his second fight at light heavyweight.
By the fight's midpoint, Parker had kept it close, causing problems for Buatsi, who utilized a tight guard to narrowly win the more effective periods.
The spectators' acceptance decreased amid constant clinching, with the challenger going down various instances during close exchanges.
The underdog finished the stronger, but multiple periods were hard to judge. Nevertheless, former world champion Carl Frampton - working with the broadcast network - had it particularly wide for Parker, awarding Buatsi just one round.
Buatsi moves on, but with the likes of power puncher David Benavidez, and Russian champion Bivol dominating the weight class, the road to world glory - judging by this showing - still appears challenging.
Hatton Memorial Motivates All-Manchester Contest
In the secondary fight, Arthur beat Rea by split-decision in a terrific all-Manchester contest to capture the continental championship.
The arena grew hushed for a profoundly moving homage ahead of the contest as 10 bells rang in remembrance of the city's native Hatton, who died 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 concluding sound faded, the silence transitioned to a roar of "only one Hatton".
The action within the ring reflected the emotion. Arthur was knocked down by a significant blow flush on the jaw in the initial stage, getting up unsteadily to his footing on the count of eight.
But the Manchester United-red attire of Arthur soon pressed ahead again. From the other corner, Rea - in opposing blue - was marked up early, significant bruising developing under his face as the pair exchanged hard punches.
The challenger, twenty-seven, connected with strong blows but Arthur, thirty-four, fought fire with fire with effective uppercuts as the action seldom decreased over the full distance.
The victor edged the decision with judges scorecards of even score, close margin and clearer margin, but both fighters were determined and courageous, all the qualities their idol Hatton stood for.
