British light heavyweight Joshua Buatsi secured a victory yet underwhelming beginning to life under new promoters with a points victory over Parker in Manchester.
The Olympic bronze medallist's punch output appeared to earn him a split decision in a close ten-round contest at the Manchester venue.
But Parker landed significant shots, delivering effective individual punches in a contest where the approaches failed to mesh, with frequent clinching marring the fight.
Two judges had it 96-94 for to Buatsi, with the remaining official a 95-95 draw.
"It wasn't my finest display but how do champions respond? They win," the victor said in the ring as he was met by boos from the audience.
The British fighter bounced back after his initial pro loss - an epic battle with Callum Smith in February - to earn a twentieth professional win.
Various analysts felt Parker, 31, should have been awarded the decision but the contender experienced the second loss of his 28-bout professional journey.
The Champion Earns Victory in Slow Encounter
The fighter's advancement has stalled in the past few years through contract problems and extended periods of inactivity.
Previously predicted as the rising talent of UK boxing, he now finds himself trying to achieve expectations - possibly in what seems like a ultimate opportunity - under recent support.
He opened tentatively, with the two boxers prepared to be aggressive in a calculated opening few rounds.
Parker, whose single earlier setback came at the lower weight class against John Ryder in 2022 after breaking his hand during the bout, was competing in only his second appearance at light heavyweight.
By the middle rounds, the underdog had made it competitive, causing problems for Buatsi, who worked behind a solid defense to barely take the better periods.
The crowd's patience decreased amid constant clinching, with the challenger going down multiple occasions during close exchanges.
The underdog ended stronger, but multiple periods were challenging to assess. Although, previous titleholder Carl Frampton - providing analysis for Dazn - had it significantly in favor for the challenger, giving Buatsi just only one stanza.
Buatsi advances, but with the presence of power puncher David Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the road to world glory - based on this performance - still appears challenging.
Ricky Hatton Memorial Motivates Local Rivalry Battle
In the supporting bout, Lyndon Arthur overcame Bradley Rea by close verdict in a outstanding Manchester vs Manchester battle to win the continental belt.
The venue grew hushed for a profoundly moving memorial ahead of the contest as a bell ceremony tolled in remembrance of the city's native Hatton, who passed away 46 years old in last month.
It was the initial significant broadcast match card in the urban center since Hatton's passing. As the final chime ended, the silence became a roar of "there's just one Hatton".
The action inside the ropes equaled the emotion. Arthur was sent to the canvas by a significant blow directly to the face in the second round, getting up unsteadily to his feet on the count of eight.
But the red-colored attire of the fighter soon advanced again. On the opposite side, the opponent - in competitive blue - was damaged quickly, noticeable damage developing under his orbital area as the fighters exchanged powerful shots.
Rea, twenty-seven, connected with strong blows but the champion, 34, matched the intensity with effective uppercuts as the pace rarely dropped over the full distance.
Arthur narrowly won the result with official scores of 114-114, close margin and clearer margin, but each competitor were determined and full of heart, exactly what their idol the legend embodied.
