British light-heavyweight Joshua Buatsi earned a win yet unimpressive beginning to life under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic bronze medallist's work rate appeared to earn him a split decision in a tight ten-round fight at the Co-op Live Arena.
But Parker found success, connecting with powerful single shots in a fight where the styles didn't complement each other, with excessive holding marring the fight.
The officials had it 96-94 for to the winner, with the final judge a 95-95.
"It wasn't my best display but what defines champions? They succeed," Buatsi stated in the ring as he was faced disapproval from the spectators.
The British fighter bounced back after his maiden career setback - an epic battle with Callum Smith in February - to earn a 20th pro success.
Some ringside observers believed the challenger, 31, should have been awarded the decision but the Derbyshire fighter endured the second loss of his 28-match professional journey.
Buatsi Secures Win in Dull Encounter
The fighter's advancement has stalled in recent times through promotional disputes and extended periods of inactivity.
Earlier forecast as the rising talent of the British fight scene, he now stands trying to realize his potential - possibly in what feels like a ultimate opportunity - under new promotional backing.
He opened tentatively, with both competitors willing to overcommit in a calculated initial rounds.
Parker, whose only previous defeat came at super-middleweight against John Ryder in two years ago after breaking his hand mid-fight, was making just his second appearance at the higher weight class.
By the fight's midpoint, Parker had kept it close, making life awkward for Buatsi, who worked behind a solid defense to barely take the more effective stanzas.
The audience's tolerance decreased because of frequent holding, with the challenger falling to the floor various instances during clinches.
Parker finished the stronger, but multiple periods were hard to judge. However, previous titleholder Carl Frampton - working with the broadcast network - had it particularly wide for Parker, giving Buatsi just one round.
The victor advances, but with the presence of hard hitter David Benavidez, and Eastern European fighter Bivol leading the category, the journey to titles - based on this performance - still looks steep.
Ricky Hatton Homage Inspires Manchester Derby Contest
In the co-main event, Lyndon Arthur defeated Rea by divided decision in a outstanding Manchester vs Manchester fight to win the continental championship.
The venue grew hushed for a profoundly moving memorial ahead of the contest as 10 bells rang in memory of the city's native Hatton, who passed away aged 46 in last month.
It was the first major televised match card in the urban center since Hatton's passing. As the concluding sound disappeared, the silence transitioned to a chant of "only one Hatton".
The action inside the ropes reflected the feeling. The eventual winner was floored by a significant blow directly to the face in the early round, getting up unsteadily to his feet on the count of eight.
But the red-colored equipment of the fighter soon advanced again. From the other corner, the opponent - in rival City blue - was marked up early, significant bruising appearing under his orbital area as the duo landed hard punches.
The challenger, 27, landed power punches but the champion, 34, fought fire with fire with effective uppercuts as the action hardly diminished over 12 rounds.
Arthur edged the result with official scores of draw, 115-113 and 115-112, but each competitor were relentless and full of heart, exactly what their idol the legend stood for.
