UK light heavyweight Joshua Buatsi made a winning yet disappointing beginning to life under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's activity level likely gained him a judges' verdict in a tight ten-round bout at the Co-op Live Arena.
But Parker found success, landing some telling individual punches in a contest where the techniques failed to mesh, with constant grabbing spoiling the contest.
Two judges judged it 96-94 for to Buatsi, with the remaining official a even score.
"It wasn't my greatest showing but what do champions do? They win," Buatsi commented in the ring as he was greeted with boos from the spectators.
The British fighter bounced back after his maiden career setback - an thrilling contest with Callum Smith in last winter - to claim a 20th career victory.
Various analysts believed the challenger, 31, should have been awarded the victory but the Derbyshire fighter experienced the second loss of his 28-fight career.
The Champion Secures Win in Uneventful Affair
The fighter's advancement has slowed in recent times through promotional disputes and considerable time of inactivity.
Previously predicted as the rising talent of British boxing, he now is positioned trying to achieve expectations - possibly in what appears to be a final roll of the dice - under recent support.
He opened tentatively, with both competitors ready to be aggressive in a tentative early stages.
Parker, whose only previous defeat came at super middleweight against John Ryder in 2022 after breaking his hand mid-fight, was making just his second bout at the higher weight class.
By the halfway stage, Parker had kept it close, causing problems for the favorite, who worked behind a protective stance to narrowly win the cleaner stanzas.
The audience's tolerance decreased during regular grabbing, with Parker falling to the floor various instances during grappling.
Parker ended stronger, but multiple periods were hard to judge. Nevertheless, former world champion the boxing analyst - providing analysis for Dazn - had it particularly wide for the challenger, giving Buatsi just only one stanza.
Buatsi advances, but with the likes of hard hitter David Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the journey to titles - based on this performance - still seems difficult.
Hatton Homage Fuels All-Manchester Battle
In the secondary fight, Lyndon Arthur overcame Rea by divided decision in a terrific local rivalry fight to secure the European light heavyweight championship.
The arena fell silent for a deeply poignant memorial ahead of the contest as 10 bells sounded in memory of the city's native Hatton, who passed away 46 years old in September.
It was the first major televised boxing event in the urban center since Hatton's death. As the concluding sound disappeared, the stillness gave way to a chant of "only one Ricky Hatton".
The fighting during the bout equaled the feeling. Arthur was sent to the canvas by a powerful punch flush on the chin in the early round, standing uncertainly to his feet on the count of eight.
But the red-colored shorts of the fighter soon came forward again. Across the ring, Rea - in competitive blue - was marked up soon, heavy swelling developing under his eye as the pair exchanged punishing blows.
Rea, twenty-seven, landed power punches but the champion, 34, matched the intensity with effective uppercuts as the intensity rarely dropped over twelve rounds.
Arthur edged the decision with judges scorecards of 114-114, narrow lead and clearer margin, but each competitor were relentless and full of heart, exactly what their inspiration the legend stood for.
