British light-heavyweight Joshua Buatsi earned a win yet underwhelming start to his career under new promoters with a points victory over Parker in Manchester.
The Olympic medal winner's activity level appeared to earn him a judges' verdict in a competitive 10-round bout at the boxing arena.
But the challenger had his moments, landing some telling single shots in a contest where the styles never quite clicked, with excessive holding spoiling the fight.
The officials judged it 96-94 to the winner, with the third a 95-95.
"It wasn't my finest display but what do champions do? They succeed," the victor stated in the ring as he was greeted with negative reactions from the crowd.
The British fighter bounced back after his initial pro loss - an epic battle with Callum Smith in last winter - to earn a 20th career victory.
Several boxing experts believed Parker, 31, deserved the victory but the British boxer suffered the second defeat of his 28-match career.
Buatsi Secures Victory in Uneventful Affair
His career development has stalled in the past few years through management issues and extended periods of inactivity.
Earlier forecast as the future of UK boxing, he now stands trying to realize his potential - possibly in what feels like a last chance - under new promotional backing.
He began cautiously, with neither fighter willing to be aggressive in a cagey early stages.
Parker, whose only previous defeat came at super-middleweight against his former opponent in 2022 after suffering a hand injury mid-fight, was competing in only his second appearance at the higher weight class.
By the middle rounds, Parker had kept it close, causing problems for Buatsi, who worked behind a tight guard to narrowly win the better rounds.
The crowd's patience diminished amid regular grabbing, with the challenger slipping to the canvas multiple occasions during grappling.
Parker finished the stronger, but multiple periods were challenging to assess. Nevertheless, previous titleholder Carl Frampton - providing analysis for the broadcast network - had it strongly leaning for the challenger, awarding Buatsi just only one stanza.
The victor advances, but with the presence of heavy hitter Benavidez, and Russian champion Bivol dominating the weight class, the road to world glory - on this evidence - still seems difficult.
Hatton Memorial Motivates All-Manchester Contest
In the supporting bout, Arthur beat Rea by divided decision in a terrific Manchester vs Manchester contest to secure the European light heavyweight belt.
The stadium grew hushed for a extremely emotional tribute ahead of the contest as a bell ceremony tolled in remembrance of Manchester's own Ricky Hatton, who succumbed at age 46 in September.
It was the premier televised fight night in the location since Hatton's death. As the final chime disappeared, the quiet transitioned to a shout of "there's just one Ricky Hatton".
The action inside the ropes reflected the sentiment. Arthur was sent to the canvas by a powerful punch directly to the chin in the initial stage, rising shakily to his footing on the count of eight.
But the traditional red equipment of the fighter soon pressed ahead again. On the opposite side, the opponent - in opposing blue - was damaged soon, heavy swelling developing under his orbital area as the fighters traded powerful shots.
The challenger, 27, delivered effective strikes but the champion, 34, responded in kind with stinging uppercuts as the pace seldom decreased over twelve rounds.
The victor narrowly won the result with scoring tallies of even score, narrow lead and wider gap, but both fighters were persistent and brave, everything their hero the legend represented.
