UK light heavyweight Joshua Buatsi secured a victory yet underwhelming beginning to life under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's activity level seemingly secured him a majority decision in a tight ten-round bout at the boxing arena.
But the challenger found success, connecting with powerful individual punches in a match where the approaches never quite clicked, with constant grabbing marring the action.
Two judges judged it 96-94 in favor to Buatsi, with the final judge a even score.
"It wasn't my best showing but how do champions respond? They win," Buatsi said in the ring as he was met by disapproval from the spectators.
The Londoner got back to winning after his initial pro loss - an epic battle with his previous opponent in February - to claim a 20th pro success.
Some ringside observers felt the challenger, thirty-one, merited the victory but the contender experienced the second defeat of his 28-fight career.
The Champion Earns Win in Uneventful Affair
The fighter's advancement has plateaued in the past few years through contract problems and considerable time of lack of fights.
Previously predicted as the next star of UK boxing, he now finds himself trying to realize his potential - maybe in what seems like a final roll of the dice - under recent support.
He began cautiously, with both competitors willing to take risks in a cagey opening few rounds.
Parker, whose single earlier setback came at super middleweight against his former opponent in 2022 after suffering a hand injury mid-fight, was competing in only his second fight at light-heavyweight.
By the middle rounds, the underdog had kept it close, creating difficulties for the favorite, who operated with a tight guard to narrowly win the better rounds.
The crowd's patience diminished amid frequent holding, with the challenger slipping to the canvas multiple occasions during grappling.
Parker closed well, but many rounds were challenging to assess. Although, ex-champion Carl Frampton - commentating on the broadcast network - had it strongly leaning for the challenger, scoring for Buatsi just one round.
The victor moves on, but with the likes of power puncher David Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the road to world glory - judging by this showing - still seems difficult.
Ricky Hatton Tribute Fuels All-Manchester Battle
In the supporting bout, Lyndon Arthur defeated Bradley Rea by split-decision in a terrific local rivalry fight to capture the continental belt.
The stadium grew hushed for a deeply poignant memorial ahead of the contest as 10 bells tolled in honor of local hero Hatton, who succumbed aged 46 in September.
It was the first major televised match card in the location since Hatton's passing. As the concluding sound ended, the silence transitioned to a chant of "there's only one Hatton".
The competition during the bout reflected the emotion. The eventual winner was knocked down by a powerful punch directly to the jaw in the early round, getting up unsteadily to his feet on the eight count.
But the red-colored attire of the fighter soon came forward again. Across the ring, the opponent - in opposing blue - was marked up soon, significant bruising forming under his eye as the fighters landed punishing blows.
Rea, twenty-seven, connected with strong blows but Arthur, thirty-four, matched the intensity with stinging uppercuts as the intensity hardly diminished over 12 rounds.
Arthur edged the decision with scoring tallies of even score, close margin and wider gap, but each competitor were persistent and brave, all the qualities their inspiration the legend stood for.
