UK light-heavyweight Joshua Buatsi earned a win yet unimpressive start to life under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic medal winner's activity level likely gained him a majority decision in a close ten-round contest at the boxing arena.
But Parker landed significant shots, landing some telling clean blows in a match where the styles never quite clicked, with excessive holding marring the contest.
Two judges had it 96-94 in favor to the winner, with the final judge a 95-95.
"It wasn't my greatest showing but how do champions respond? They prevail," Buatsi stated in the arena as he was greeted with boos from the spectators.
The Londoner bounced back after his first professional defeat - an epic battle with Callum Smith in last winter - to secure a twentieth professional win.
Various analysts felt the challenger, 31, should have been awarded the victory but the Derbyshire fighter suffered the second setback of his 28-bout professional journey.
The Champion Earns Victory in Dull Match
The fighter's advancement has slowed in the past few years through management issues and extended periods of inactivity.
Earlier forecast as the rising talent of British boxing, he now finds himself trying to achieve expectations - maybe in what feels like a last chance - under new promotional backing.
He started carefully, with both competitors ready to take risks in a cagey opening few rounds.
Parker, whose sole prior loss came at the lower weight class against his former opponent in two years ago after suffering a hand injury in the contest, was participating in his second appearance at the higher weight class.
By the fight's midpoint, the underdog had kept it close, making life awkward for Buatsi, who worked behind a tight guard to narrowly win the cleaner stanzas.
The crowd's patience diminished amid frequent holding, with the challenger slipping to the canvas multiple occasions during close exchanges.
Parker closed well, but multiple periods were challenging to assess. Nevertheless, former world champion the boxing analyst - commentating on Dazn - had it significantly in favor for Parker, awarding Buatsi just a single round.
The victor progresses, but with the caliber of power puncher David Benavidez, and Eastern European fighter Bivol ruling the division, the path to championship success - judging by this showing - still appears challenging.
Hatton Tribute Motivates Local Rivalry Fight
In the secondary fight, Arthur overcame Rea by close verdict in a terrific all-Manchester contest to secure the continental title.
The venue grew hushed for a deeply poignant homage ahead of the contest as 10 bells rang in honor of local hero Ricky Hatton, who succumbed aged 46 in September.
It was the first major televised match card in the location since Hatton's death. As the concluding sound faded, the quiet became a shout of "there's just one Hatton".
The competition within the ring reflected the feeling. Arthur was floored by a left hook squarely on the chin in the initial stage, rising shakily to his footing on the referee's count.
But the traditional red equipment of the fighter soon came forward again. Across the ring, the opponent - in competitive blue - was damaged soon, heavy swelling appearing under his face as the fighters exchanged punishing blows.
Rea, 27, delivered effective strikes but Arthur, thirty-four, matched the intensity with powerful upward punches as the action rarely dropped over the full distance.
Arthur narrowly won the result with scoring tallies of even score, 115-113 and 115-112, but the two athletes were persistent and brave, everything their idol the legend represented.
