UK light heavyweight Joshua Buatsi earned a win yet disappointing beginning to his career under new promoters with a decision win over Parker in the Manchester arena.
The Olympic medal winner's work rate seemingly secured him a judges' verdict in a tight 10-round fight at the Manchester venue.
But the challenger had his moments, landing some telling clean blows in a match where the approaches didn't complement each other, with frequent clinching marring the fight.
The scoring judges had it 96-94 for to the winner, with the third a even score.
"It wasn't my finest display but how do champions respond? They win," the victor stated in the ringside as he was greeted with boos from the audience.
The Londoner bounced back after his first professional defeat - an thrilling contest with his previous opponent in February - to secure a twentieth professional win.
Several boxing experts believed Parker, 31, merited the victory but the Derbyshire fighter experienced the second setback of his 28-bout professional journey.
Buatsi Claims Win in Slow Encounter
His career development has stalled in recent times through contract problems and considerable time of inactivity.
Earlier forecast as the next star of the British fight scene, he now finds himself trying to achieve expectations - perhaps in what seems like a last chance - under new promotional backing.
He started carefully, with the two boxers prepared to be aggressive in a cagey initial rounds.
The challenger, whose only previous defeat came at super middleweight against John Ryder in 2022 after breaking his hand in the contest, was making just his second fight at light heavyweight.
By the middle rounds, the underdog had kept it close, creating difficulties for Buatsi, who operated with a tight guard to edge the better periods.
The crowd's patience wore thin during constant clinching, with Parker going down several times during grappling.
Parker finished the stronger, but many rounds were hard to judge. However, former world champion the boxing analyst - working with the broadcast network - had it strongly leaning for the challenger, giving Buatsi just a single round.
The victor advances, but with the presence of heavy hitter David Benavidez, and Russian star Bivol dominating the weight class, the path to championship success - on this evidence - still seems difficult.
Ricky Hatton Homage Motivates Local Rivalry Battle
In the co-main event, Lyndon Arthur overcame Bradley Rea by divided decision in a terrific Manchester vs Manchester battle to win the European light heavyweight title.
The arena became quiet for a deeply poignant homage before the fight as ten bells rang in memory of the city's native Ricky Hatton, who succumbed at age 46 in last month.
It was the first major televised boxing event in the urban center since Hatton's death. As the concluding sound disappeared, the stillness transitioned to a chant of "only one Ricky Hatton".
The action during the bout equaled the emotion. The eventual winner was sent to the canvas by a powerful punch flush on the jaw in the second round, getting up unsteadily to his footing on the count of eight.
But the Manchester United-red equipment of the fighter soon came forward again. From the other corner, Rea - in competitive blue - was marked up soon, heavy swelling forming under his eye as the duo traded hard punches.
The challenger, twenty-seven, connected with strong blows but the champion, 34, matched the intensity with effective uppercuts as the pace hardly diminished over 12 rounds.
Arthur edged the verdict with scoring tallies of 114-114, 115-113 and clearer margin, but each competitor were relentless and brave, everything their idol Hatton embodied.
