British light heavyweight Joshua Buatsi made a winning yet underwhelming start to life under fresh promotional backing with a points victory over Parker in the Manchester arena.
The Olympic medal winner's punch output appeared to earn him a majority decision in a competitive 10-round fight at the Co-op Live Arena.
But Parker found success, delivering effective single shots in a contest where the styles failed to mesh, with frequent clinching disrupting the action.
Two judges scored it 96-94 to Buatsi, with the final judge a 95-95.
"It wasn't my finest display but what defines champions? They prevail," the victor commented in the ringside as he was met by disapproval from the audience.
The Londoner got back to winning after his initial pro loss - an epic battle with Callum Smith in last winter - to earn a twentieth professional win.
Various analysts believed Parker, 31, merited the victory but the Derbyshire fighter experienced the second defeat of his 28-bout professional journey.
The Champion Claims Triumph in Slow Encounter
Buatsi's progress has stalled in recent years through management issues and extended periods of inactivity.
Previously predicted as the rising talent of the British fight scene, he now stands trying to achieve expectations - maybe in what appears to be a final roll of the dice - under recent support.
He began cautiously, with both competitors ready to take risks in a cagey early stages.
Parker, whose single earlier setback came at super middleweight against his former opponent in 2022 after damaging his hand in the contest, was participating in his second fight at the higher weight class.
By the middle rounds, Parker had made it competitive, creating difficulties for the favorite, who operated with a protective stance to barely take the more effective stanzas.
The spectators' acceptance wore thin because of constant clinching, with the challenger going down multiple occasions during clinches.
The underdog finished the stronger, but several stanzas were hard to judge. Nevertheless, former world champion Carl Frampton - working with the broadcast network - had it strongly leaning for the challenger, scoring for Buatsi just one round.
The victor advances, but with the presence of power puncher Benavidez, and Eastern European fighter Bivol dominating the weight class, the journey to titles - judging by this showing - still looks steep.
Ricky Hatton Homage Motivates All-Manchester Fight
In the supporting bout, Lyndon Arthur beat Rea by divided decision in a outstanding Manchester vs Manchester battle to secure the European light-heavyweight belt.
The arena became quiet for a deeply poignant memorial ahead of the contest as ten bells tolled in honor of the city's native Hatton, who succumbed at age 46 in recently.
It was the first major televised fight night in the city since Hatton's passing. As the final chime disappeared, the quiet gave way to a chant of "there's only one Ricky Hatton".
The fighting during the bout matched the feeling. Arthur was knocked down by a left hook directly to the face in the early round, standing uncertainly to his footing on the referee's count.
But the red-colored attire of Arthur soon pressed ahead again. Across the ring, the opponent - in competitive blue - was injured quickly, noticeable damage appearing under his face as the pair landed punishing blows.
Rea, 27, connected with strong blows but Arthur, thirty-four, responded in kind with stinging uppercuts as the intensity hardly diminished over 12 rounds.
Arthur narrowly won the verdict with scoring tallies of even score, close margin and clearer margin, but each competitor were determined and courageous, everything their hero Hatton represented.
