British light-heavyweight Joshua Buatsi made a winning yet disappointing beginning to his career under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's punch output appeared to earn him a majority decision in a competitive 10-round fight at the Manchester venue.
But the challenger landed significant shots, delivering effective individual punches in a match where the techniques failed to mesh, with constant grabbing marring the fight.
Two judges judged it 96-94 for to Buatsi, with the third a 95-95.
"It wasn't my finest performance but what defines champions? They prevail," the victor stated in the arena as he was greeted with boos from the spectators.
The British fighter got back to winning after his maiden career setback - an memorable fight with his previous opponent in February - to claim a twentieth professional win.
Some ringside observers thought Parker, 31, deserved the decision but the Derbyshire fighter suffered the second setback of his 28-bout professional journey.
The Champion Earns Win in Uneventful Encounter
The fighter's advancement has stalled in recent years through management issues and long spells of lack of fights.
Previously predicted as the next star of UK boxing, he now finds himself trying to achieve expectations - maybe in what seems like a final roll of the dice - under new promotional backing.
He opened tentatively, with the two boxers prepared to overcommit in a calculated initial rounds.
The challenger, whose only previous defeat came at super-middleweight against his former opponent in two years ago after suffering a hand injury during the bout, was competing in only his second appearance at light heavyweight.
By the halfway stage, the underdog had kept it close, causing problems for Buatsi, who operated with a protective stance to barely take the better stanzas.
The audience's tolerance decreased during frequent holding, with Parker slipping to the canvas multiple occasions during grappling.
The underdog ended stronger, but multiple periods were hard to judge. Nevertheless, ex-champion Carl Frampton - working with Dazn - had it significantly in favor for Parker, giving Buatsi just one round.
The victor moves on, but with the caliber of heavy hitter David Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the path to championship success - judging by this showing - still looks steep.
Hatton Memorial Inspires Manchester Derby Battle
In the secondary fight, Arthur overcame Rea by close verdict in a outstanding all-Manchester fight to secure the European light heavyweight title.
The arena grew hushed for a extremely emotional memorial before the fight as a bell ceremony rang in honor of Manchester's own Hatton, who passed away aged 46 in recently.
It was the initial significant broadcast match card in the city since Hatton's passing. As the final chime disappeared, the stillness transitioned to a roar of "there's just one Ricky Hatton".
The fighting inside the ropes matched the feeling. Arthur was knocked down by a significant blow flush on the face in the second round, getting up unsteadily to his footing on the referee's count.
But the traditional red attire of the fighter soon advanced again. From the other corner, Rea - in rival City blue - was damaged quickly, heavy swelling developing under his orbital area as the fighters exchanged punishing blows.
Rea, twenty-seven, delivered effective strikes but the champion, 34, fought fire with fire with effective uppercuts as the action seldom decreased over 12 rounds.
The victor edged the result with official scores of even score, 115-113 and clearer margin, but the two athletes were relentless and full of heart, all the qualities their inspiration the legend represented.
