UK light heavyweight Joshua Buatsi earned a win yet unimpressive start to life under new promoters with a points victory over Parker in the Manchester arena.
The Olympic medal winner's punch output likely gained him a judges' verdict in a tight 10-round fight at the Co-op Live Arena.
But Parker found success, landing some telling single shots in a fight where the techniques failed to mesh, with frequent clinching disrupting the contest.
Two judges had it 96-94 for to Buatsi, with the final judge a 95-95.
"It wasn't my finest performance but how do champions respond? They win," Buatsi said in the ring as he was faced boos from the crowd.
The British fighter got back to winning after his first professional defeat - an thrilling contest with Callum Smith in February - to earn a 20th pro success.
Several boxing experts felt Parker, thirty-one, deserved the victory but the British boxer endured the second defeat of his 28-fight professional journey.
Buatsi Earns Win in Dull Encounter
Buatsi's progress has stalled in recent years through promotional disputes and considerable time of inactivity.
Earlier forecast as the next star of the British fight scene, he now is positioned trying to realize his potential - perhaps in what appears to be a final roll of the dice - under recent support.
He opened tentatively, with neither fighter willing to take risks in a tentative initial rounds.
Parker, whose single earlier setback came at super-middleweight against John Ryder in two years ago after breaking his hand mid-fight, was making just his second fight at the higher weight class.
By the halfway stage, the underdog had stayed in contention, making life awkward for the favorite, who worked behind a tight guard to edge the more effective stanzas.
The crowd's patience wore thin amid constant clinching, with the challenger falling to the floor multiple occasions during close exchanges.
Parker finished the stronger, but multiple periods were challenging to assess. Nevertheless, former world champion Carl Frampton - working with Dazn - had it particularly wide for the challenger, giving Buatsi just one round.
The victor moves on, but with the caliber of power puncher David Benavidez, and Russian star Dmitry Bivol leading the category, the road to world glory - on this evidence - still looks steep.
Ricky Hatton Memorial Motivates All-Manchester Contest
In the supporting bout, Arthur overcame Rea by split-decision in a outstanding all-Manchester battle to secure the European light-heavyweight title.
The venue became quiet for a profoundly moving tribute ahead of the contest as ten bells sounded in memory of local hero Ricky Hatton, who passed away 46 years old in recently.
It was the initial significant broadcast fight night in the city since Hatton's passing. As the concluding sound disappeared, the stillness transitioned to a chant of "only one Hatton".
The fighting within the ring reflected the emotion. Arthur was knocked down by a significant blow squarely on the jaw in the early round, standing uncertainly to his feet on the count of eight.
But the traditional red shorts of Arthur soon pressed ahead again. On the opposite side, Rea - in competitive blue - was marked up soon, noticeable damage developing under his face as the pair traded punishing blows.
Rea, 27, connected with strong blows but the champion, thirty-four, matched the intensity with stinging uppercuts as the intensity rarely dropped over twelve rounds.
Arthur narrowly won the verdict with scoring tallies of even score, 115-113 and wider gap, but each competitor were determined and full of heart, all the qualities their idol the legend represented.
