UK light-heavyweight Joshua Buatsi earned a win yet disappointing start to his career under fresh promotional backing with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's punch output likely gained him a judges' verdict in a tight 10-round contest at the boxing arena.
But the challenger landed significant shots, delivering effective single shots in a match where the styles didn't complement each other, 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 greatest performance but how do champions respond? They succeed," Buatsi commented in the ring as he was greeted with boos from the spectators.
The Londoner returned to winning ways after his maiden career setback - an memorable fight with Callum Smith in February - to secure a twentieth professional win.
Some ringside observers believed Parker, thirty-one, should have been awarded the win but the British boxer experienced the second setback of his 28-fight career.
The Champion Secures Win in Slow Match
Buatsi's progress has plateaued in recent years through promotional disputes and extended periods of inactivity.
Earlier forecast as the future of British boxing, he now finds himself trying to achieve expectations - perhaps in what feels like a final roll of the dice - under fresh management.
He began cautiously, with the two boxers ready to take risks in a calculated initial rounds.
The challenger, whose single earlier setback came at super-middleweight against John Ryder in 2022 after suffering a hand injury mid-fight, was competing in only his second bout at light heavyweight.
By the fight's midpoint, the underdog had made it competitive, creating difficulties for the favorite, who worked behind a protective stance to barely take the more effective stanzas.
The crowd's patience decreased during frequent holding, with Parker slipping to the canvas several times during close exchanges.
The underdog finished the stronger, but many rounds were hard to judge. Although, previous titleholder Carl Frampton - providing analysis for the broadcast network - had it strongly leaning for the challenger, giving Buatsi just one round.
The victor moves on, but with the likes of heavy hitter Benavidez, and Russian star Bivol ruling the division, the path to championship success - on this evidence - still seems difficult.
Hatton Homage Fuels All-Manchester Contest
In the secondary fight, Arthur beat Bradley Rea by split-decision in a outstanding Manchester vs Manchester fight to secure the European light-heavyweight championship.
The stadium grew hushed for a extremely emotional memorial ahead of the contest as ten bells sounded in remembrance of Manchester's own Ricky Hatton, who passed away aged 46 in September.
It was the initial significant broadcast match card in the urban center since Hatton's death. As the concluding sound disappeared, the quiet gave way to a roar of "there's only one Ricky Hatton".
The action within the ring reflected the sentiment. Arthur was knocked down by a powerful punch flush on the jaw in the early round, standing uncertainly to his feet on the count of eight.
But the Manchester United-red equipment of the fighter soon came forward again. From the other corner, Rea - in rival City blue - was marked up quickly, heavy swelling appearing under his face as the duo landed powerful shots.
Rea, twenty-seven, connected with strong blows but Arthur, 34, matched the intensity with stinging uppercuts as the intensity rarely dropped over twelve rounds.
The victor barely secured the verdict with official scores of even score, 115-113 and clearer margin, but both fighters were persistent and full of heart, all the qualities their idol the legend embodied.
