UK light-heavyweight Joshua Buatsi secured a victory yet underwhelming beginning to life under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic bronze medallist's punch output appeared to earn him a split decision in a close ten-round fight at the Co-op Live Arena.
But Parker found success, connecting with powerful single shots in a match where the styles failed to mesh, with constant grabbing marring the contest.
The scoring judges scored it 96-94 to the winner, with the third a 95-95.
"It wasn't my finest showing but what defines champions? They succeed," Buatsi commented in the ring as he was greeted with boos from the spectators.
The British fighter bounced back after his initial pro loss - an thrilling contest with his previous opponent in February - to earn a 20th pro success.
Several boxing experts felt the challenger, thirty-one, deserved the win but the contender endured the second setback of his 28-fight professional journey.
The Champion Secures Win in Uneventful Encounter
Buatsi's progress has slowed in recent years through promotional disputes and long spells of lack of fights.
Once tipped as the future of the British fight scene, he now finds himself trying to achieve expectations - possibly in what seems like a ultimate opportunity - under new promotional backing.
He began cautiously, with the two boxers prepared to take risks in a calculated initial rounds.
Parker, whose only previous defeat came at the lower weight class against his former opponent in two years ago after suffering a hand injury in the contest, was competing in only his second bout at light-heavyweight.
By the middle rounds, Parker had stayed in contention, making life awkward for Buatsi, who worked behind a tight guard to edge the better stanzas.
The audience's tolerance decreased because of frequent holding, with Parker slipping to the canvas various instances during clinches.
The underdog closed well, but multiple periods were hard to judge. Nevertheless, previous titleholder the boxing analyst - providing analysis for Dazn - had it strongly leaning for Parker, giving Buatsi just a single round.
Buatsi progresses, but with the presence of hard hitter David Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the path to championship success - judging by this showing - still looks steep.
Ricky Hatton Memorial Fuels Local Rivalry Contest
In the secondary fight, Lyndon Arthur beat Rea by split-decision in a excellent Manchester vs Manchester battle to win the European light heavyweight championship.
The venue fell silent for a deeply poignant tribute ahead of the contest as ten bells tolled in memory of the city's native Ricky Hatton, who succumbed aged 46 in September.
It was the premier televised match card in the urban center since Hatton's passing. As the final chime disappeared, the stillness gave way to a shout of "there's only one Hatton".
The competition inside the ropes equaled the feeling. The eventual winner was knocked down by a powerful punch squarely on the chin in the initial stage, standing uncertainly to his footing on the eight count.
But the Manchester United-red shorts of Arthur soon came forward again. From the other corner, Rea - in rival City blue - was injured soon, heavy swelling forming under his eye as the fighters landed punishing blows.
Rea, 27, connected with strong blows but Arthur, thirty-four, fought fire with fire with stinging uppercuts as the action seldom decreased over the full distance.
Arthur barely secured the result with scoring tallies of even score, close margin and wider gap, but both fighters were relentless and full of heart, all the qualities their hero the legend represented.
