UK light-heavyweight Joshua Buatsi earned a win yet disappointing start to his career under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's work rate appeared to earn him a split decision in a competitive ten-round contest at the boxing arena.
But Parker landed significant shots, connecting with powerful clean blows in a match where the approaches didn't complement each other, with constant grabbing spoiling the contest.
Two judges had it 96-94 for to Buatsi, with the third a 95-95.
"It wasn't my finest showing but what defines champions? They win," Buatsi stated in the ringside as he was greeted with disapproval from the crowd.
The British fighter returned to winning ways after his first professional defeat - an thrilling contest with his previous opponent in February - to secure a 20th career victory.
Various analysts believed the challenger, thirty-one, merited the decision but the British boxer experienced the second loss of his 28-match career.
Buatsi Secures Triumph in Uneventful Match
The fighter's advancement has slowed in the past few years through promotional disputes and extended periods of inactivity.
Previously predicted as the rising talent of the British fight scene, he now is positioned trying to realize his potential - possibly in what feels like a final roll of the dice - under recent support.
He started carefully, with the two boxers prepared to overcommit in a calculated opening few rounds.
Parker, whose single earlier setback came at super-middleweight against John Ryder in two years ago after damaging his hand in the contest, was competing in only his second fight at light heavyweight.
By the halfway stage, Parker had stayed in contention, creating difficulties for Buatsi, who operated with a solid defense to narrowly win the more effective rounds.
The crowd's patience diminished amid regular grabbing, with the challenger going down various instances during close exchanges.
Parker ended stronger, but several stanzas were difficult to score. Although, ex-champion the boxing analyst - providing analysis for Dazn - had it particularly wide for Parker, awarding Buatsi just only one stanza.
Buatsi progresses, but with the presence of hard hitter David Benavidez, and Russian star Dmitry Bivol leading the category, the path to championship success - judging by this showing - still looks steep.
Hatton Memorial Inspires Local Rivalry Contest
In the co-main event, Lyndon Arthur defeated Bradley Rea by split-decision in a excellent all-Manchester contest to capture the continental championship.
The stadium became quiet for a extremely emotional memorial before the fight as ten bells sounded in remembrance of local hero Hatton, who died at age 46 in September.
It was the initial significant broadcast match card in the urban center since the passing of Hatton. As the concluding sound faded, the stillness transitioned to a chant of "only one Hatton".
The competition during the bout matched the emotion. The eventual winner was sent to the canvas by a powerful punch directly to the chin in the early round, standing uncertainly to his footing on the eight count.
But the Manchester United-red shorts of Arthur soon pressed ahead again. From the other corner, Rea - in competitive blue - was marked up soon, noticeable damage developing under his eye as the duo exchanged powerful shots.
The challenger, 27, delivered effective strikes but the champion, thirty-four, matched the intensity with stinging uppercuts as the intensity seldom decreased over twelve rounds.
Arthur barely secured the decision with judges scorecards of 114-114, 115-113 and clearer margin, but both fighters were determined and full of heart, everything their inspiration the legend embodied.
