British light-heavyweight Joshua Buatsi secured a victory yet unimpressive start to his career under new promoters with a points victory over Parker in Manchester.
The Olympic bronze medallist's work rate appeared to earn him a majority decision in a close ten-round bout at the Manchester venue.
But Parker had his moments, landing some telling individual punches in a match where the styles failed to mesh, with frequent clinching spoiling the action.
The officials had it 96-94 for to the winner, with the third a even score.
"It wasn't my greatest display but what defines champions? They prevail," the victor stated in the ring as he was faced negative reactions from the crowd.
The British fighter bounced back after his first professional defeat - an thrilling contest with Callum Smith in February - to secure a twentieth professional win.
Various analysts thought the challenger, thirty-one, should have been awarded the decision but the contender endured the second defeat of his 28-fight professional journey.
The Champion Secures Triumph in Dull Affair
Buatsi's progress has stalled in recent times through management issues and extended periods of inactivity.
Previously predicted as the next star of British boxing, he now stands trying to fulfil that early promise - maybe in what seems like a final roll of the dice - under new promotional backing.
He started carefully, with both competitors willing to be aggressive in a tentative early stages.
The challenger, whose single earlier setback came at super middleweight against his former opponent in 2022 after suffering a hand injury mid-fight, was making just his second bout at light heavyweight.
By the fight's midpoint, the underdog had stayed in contention, creating difficulties for Buatsi, who worked behind a protective stance to narrowly win the cleaner periods.
The audience's tolerance decreased amid constant clinching, with Parker falling to the floor various instances during grappling.
The underdog ended stronger, but multiple periods were challenging to assess. However, previous titleholder Carl Frampton - working with Dazn - had it particularly wide for Parker, awarding Buatsi just one round.
The victor moves on, but with the presence of power puncher David Benavidez, and Russian star Dmitry Bivol leading the category, the road to world glory - judging by this showing - still seems difficult.
Hatton Tribute Fuels Manchester Derby Battle
In the supporting bout, Lyndon Arthur defeated Rea by split-decision in a terrific all-Manchester battle to capture the European light-heavyweight title.
The venue fell silent for a extremely emotional memorial ahead of the contest as 10 bells tolled in remembrance of Manchester's own Hatton, who died at age 46 in September.
It was the first major televised match card in the city since Hatton's death. As the concluding sound disappeared, the stillness transitioned to a chant of "there's only one Ricky Hatton".
The fighting during the bout matched the feeling. Arthur was floored by a left hook directly to the jaw in the early round, standing uncertainly to his position on the eight count.
But the red-colored equipment of Arthur soon advanced again. Across the ring, Rea - in competitive blue - was damaged soon, noticeable damage forming under his face as the fighters traded powerful shots.
Rea, 27, connected with strong blows but the champion, thirty-four, responded in kind with stinging uppercuts as the action seldom decreased over 12 rounds.
The victor narrowly won the result with judges scorecards of even score, close margin and clearer margin, but each competitor were relentless and courageous, exactly what their hero Hatton represented.
