UK light-heavyweight Joshua Buatsi secured a victory yet disappointing start to his career under recent management with a points victory over Parker in Manchester.
The Olympic bronze medallist's activity level likely gained him a majority decision in a tight ten-round fight at the Manchester venue.
But Parker found success, connecting with powerful clean blows in a fight where the approaches never quite clicked, with excessive holding marring the contest.
The officials had it 96-94 in favor to Buatsi, with the remaining official a even score.
"It wasn't my greatest performance but what do champions do? They succeed," the victor said in the arena as he was greeted with negative reactions from the crowd.
The British fighter returned to winning ways after his maiden career setback - an epic battle with Callum Smith in February - to secure a twentieth professional win.
Some ringside observers thought the challenger, thirty-one, deserved the victory but the contender suffered the second defeat of his 28-fight professional journey.
The Champion Claims Win in Slow Affair
His career development has stalled in recent times through promotional disputes and considerable time of inactivity.
Previously predicted as the future of British boxing, he now finds himself trying to achieve expectations - maybe in what appears to be a final roll of the dice - under fresh management.
He began cautiously, with neither fighter ready to be aggressive in a calculated opening few rounds.
Parker, whose only previous defeat came at the lower weight class against his former opponent in two years ago after damaging his hand in the contest, was participating in his second appearance at light-heavyweight.
By the halfway stage, the underdog had made it competitive, creating difficulties for the favorite, who worked behind a tight guard to narrowly win the more effective rounds.
The audience's tolerance wore thin during regular grabbing, with the challenger going down various instances during clinches.
The underdog ended stronger, but many rounds were hard to judge. However, previous titleholder Carl Frampton - providing analysis for Dazn - had it significantly in favor for Parker, scoring for Buatsi just one round.
Buatsi advances, but with the caliber of power puncher David Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the journey to titles - based on this performance - still appears challenging.
Ricky Hatton Memorial Fuels All-Manchester Contest
In the co-main event, Arthur overcame Rea by split-decision in a outstanding Manchester vs Manchester contest to win the European light heavyweight title.
The venue became quiet for a profoundly moving homage ahead of the contest as a bell ceremony rang in remembrance of the city's native Hatton, who died at age 46 in September.
It was the initial significant broadcast fight night in the location since the passing of Hatton. As the final chime ended, the stillness gave way to a shout of "there's only one Hatton".
The action within the ring reflected the sentiment. The eventual winner was sent to the canvas by a powerful punch directly to the face in the second round, getting up unsteadily to his feet on the count of eight.
But the traditional red shorts of the fighter soon came forward again. Across the ring, the opponent - in opposing blue - was marked up early, noticeable damage forming under his orbital area as the pair exchanged powerful shots.
Rea, 27, landed power punches but Arthur, thirty-four, responded in kind with stinging uppercuts as the intensity hardly diminished over 12 rounds.
Arthur edged the verdict with scoring tallies of even score, 115-113 and clearer margin, but each competitor were relentless and brave, exactly what their hero Hatton stood for.
