British light heavyweight Joshua Buatsi made a winning yet disappointing start to life under fresh promotional backing with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's work rate likely gained him a majority decision in a close ten-round contest at the Manchester venue.
But the challenger found success, delivering effective individual punches in a contest where the styles didn't complement each other, with frequent clinching marring the fight.
The scoring judges had it 96-94 to Buatsi, with the remaining official a 95-95.
"It wasn't my best showing but what do champions do? They win," the victor stated in the arena as he was faced negative reactions from the spectators.
The Londoner bounced back after his initial pro loss - an epic battle with Callum Smith in last winter - to claim a twentieth professional win.
Several boxing experts believed the challenger, 31, deserved the victory but the contender experienced the second loss of his 28-match career.
The Champion Claims Triumph in Slow Encounter
Buatsi's progress has slowed in the past few years through contract problems and extended periods of inactivity.
Earlier forecast as the next star of British boxing, he now finds himself trying to realize his potential - maybe in what appears to be a last chance - under new promotional backing.
He started carefully, with both competitors prepared to overcommit in a tentative early stages.
Parker, whose sole prior loss came at super middleweight against his former opponent in 2022 after breaking his hand mid-fight, was participating in his second appearance at the higher weight class.
By the fight's midpoint, Parker had made it competitive, causing problems for Buatsi, who operated with a protective stance to barely take the more effective stanzas.
The audience's tolerance diminished amid frequent holding, with the challenger slipping to the canvas various instances during clinches.
Parker finished the stronger, but several stanzas were hard to judge. However, former world champion Carl Frampton - working with Dazn - had it strongly leaning for the challenger, awarding Buatsi just only one stanza.
Buatsi moves on, but with the presence of heavy hitter Benavidez, and Russian champion Bivol dominating the weight class, the journey to titles - judging by this showing - still looks steep.
Hatton Memorial Inspires All-Manchester Battle
In the supporting bout, Lyndon Arthur beat Bradley Rea by divided decision in a excellent all-Manchester battle to secure the continental title.
The arena became quiet for a profoundly moving memorial before the fight as 10 bells tolled in honor of the city's native Hatton, who passed away at age 46 in September.
It was the premier televised boxing event in the urban center since the passing of Hatton. As the last bell disappeared, the stillness became a chant of "only one Hatton".
The competition within the ring matched the feeling. The eventual winner was sent to the canvas by a powerful punch flush on the jaw in the second round, rising shakily to his feet on the referee's count.
But the Manchester United-red equipment of Arthur soon pressed ahead again. On the opposite side, Rea - in rival City blue - was damaged soon, heavy swelling forming under his orbital area as the fighters landed hard punches.
The challenger, 27, connected with strong blows but the champion, thirty-four, fought fire with fire with effective uppercuts as the pace hardly diminished over twelve rounds.
Arthur narrowly won the verdict with judges scorecards of 114-114, 115-113 and clearer margin, but the two athletes were persistent and full of heart, everything their inspiration Hatton embodied.
