UK light heavyweight Joshua Buatsi earned a win yet disappointing beginning to life under recent management with a decision win over Parker in Manchester.
The Olympic medal winner's activity level likely gained him a split decision in a tight 10-round bout at the boxing arena.
But Parker landed significant shots, connecting with powerful clean blows in a fight where the styles didn't complement each other, with constant grabbing marring the fight.
The officials had it 96-94 to Buatsi, with the final judge a 95-95 draw.
"It wasn't my greatest showing but what defines champions? They win," Buatsi commented in the ringside as he was greeted with negative reactions from the audience.
The Londoner bounced back after his maiden career setback - an epic battle with his previous opponent in February - to secure a twentieth professional win.
Several boxing experts believed the challenger, thirty-one, deserved the decision but the contender suffered the second defeat of his 28-fight career.
The Champion Earns Win in Slow Encounter
Buatsi's progress has slowed in the past few years through contract problems and long spells of lack of fights.
Once tipped as the rising talent of British boxing, he now finds himself trying to realize his potential - perhaps in what seems like a ultimate opportunity - under recent support.
He started carefully, with the two boxers willing to take risks in a cagey initial rounds.
The challenger, whose single earlier setback came at super-middleweight against his former opponent in two years ago after damaging his hand in the contest, was making just his second bout at light heavyweight.
By the middle rounds, the underdog had made it competitive, causing problems for the favorite, who utilized a solid defense to edge the more effective periods.
The spectators' acceptance wore thin because of regular grabbing, with Parker falling to the floor multiple occasions during close exchanges.
The underdog closed well, but multiple periods were hard to judge. Although, ex-champion the boxing analyst - providing analysis for Dazn - had it particularly wide for Parker, scoring for Buatsi just only one stanza.
Buatsi progresses, but with the caliber of heavy hitter David Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the journey to titles - based on this performance - still appears challenging.
Hatton Homage Inspires All-Manchester Fight
In the co-main event, Arthur defeated Rea by close verdict in a terrific all-Manchester battle to win the continental championship.
The venue grew hushed for a extremely emotional memorial ahead of the contest as 10 bells rang in remembrance of local hero Ricky Hatton, who passed away at age 46 in recently.
It was the initial significant broadcast boxing event in the location since Hatton's passing. As the concluding sound ended, the stillness gave way to a chant of "there's only one Hatton".
The competition inside the ropes matched the emotion. The eventual winner was knocked down by a powerful punch squarely on the chin in the second round, getting up unsteadily to his footing on the eight count.
But the Manchester United-red equipment of Arthur soon came forward again. On the opposite side, the opponent - in competitive blue - was injured quickly, heavy swelling developing under his orbital area as the fighters landed hard punches.
Rea, twenty-seven, delivered effective strikes but the champion, 34, responded in kind with powerful upward punches as the action seldom decreased over the full distance.
The victor edged the verdict with official scores of 114-114, close margin and clearer margin, but the two athletes were relentless and brave, all the qualities their inspiration the legend represented.
