British light heavyweight Joshua Buatsi made a winning yet disappointing debut to life under new promoters with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's activity level seemingly secured him a judges' verdict in a competitive 10-round bout at the boxing arena.
But the challenger had his moments, delivering effective individual punches in a contest where the approaches didn't complement each other, with frequent clinching marring the contest.
The officials judged it 96-94 to the winner, with the remaining official a even score.
"It wasn't my finest display but how do champions respond? They succeed," Buatsi stated in the arena as he was greeted with disapproval from the audience.
The Londoner returned to winning ways after his initial pro loss - an thrilling contest with Callum Smith in February - to claim a 20th career victory.
Various analysts believed Parker, thirty-one, merited the victory but the contender suffered the second setback of his 28-fight career.
Buatsi Secures Triumph in Slow Encounter
Buatsi's progress has slowed in the past few years through contract problems and extended periods of inactivity.
Previously predicted as the rising talent of the British fight scene, he now stands trying to realize his potential - possibly in what feels like a ultimate opportunity - under recent support.
He started carefully, with neither fighter willing to overcommit in a calculated opening few rounds.
Parker, whose only previous defeat came at the lower weight class against John Ryder in two years ago after damaging his hand in the contest, was participating in his second appearance at light heavyweight.
By the fight's midpoint, Parker had stayed in contention, causing problems for the favorite, who worked behind a tight guard to narrowly win the better periods.
The audience's tolerance decreased during constant clinching, with the challenger going down multiple occasions during grappling.
Parker finished the stronger, but many rounds were challenging to assess. Nevertheless, previous titleholder the boxing analyst - working with Dazn - had it strongly leaning for Parker, awarding Buatsi just one round.
The victor progresses, but with the caliber of power puncher David Benavidez, and Russian star Dmitry Bivol ruling the division, the road to world glory - on this evidence - still looks steep.
Hatton Memorial Inspires Local Rivalry Contest
In the co-main event, Lyndon Arthur overcame Bradley Rea by divided decision in a terrific Manchester vs Manchester fight to capture the European light heavyweight title.
The arena became quiet for a profoundly moving tribute before the fight as 10 bells rang in remembrance of the city's native Hatton, who died at age 46 in last month.
It was the initial significant broadcast boxing event in the city since the passing of Hatton. As the concluding sound faded, the quiet became a chant of "there's only one Ricky Hatton".
The competition during the bout equaled the sentiment. The eventual winner was floored by a left hook squarely on the face in the initial stage, standing uncertainly to his feet on the count of eight.
But the traditional red attire of Arthur soon came forward again. From the other corner, the opponent - in competitive blue - was damaged early, noticeable damage developing under his eye as the duo landed powerful shots.
Rea, 27, delivered effective strikes but Arthur, 34, responded in kind with powerful upward punches as the pace hardly diminished over the full distance.
Arthur barely secured the result with scoring tallies of draw, close margin and clearer margin, but each competitor were relentless and courageous, exactly what their idol Hatton represented.
