British light-heavyweight Joshua Buatsi made a winning yet unimpressive beginning to life under recent management with a points victory over Parker in Manchester.
The Olympic medal winner's work rate appeared to earn him a split decision in a competitive 10-round fight at the Co-op Live Arena.
But Parker found success, connecting with powerful single shots in a contest where the approaches failed to mesh, with constant grabbing marring the contest.
Two judges scored it 96-94 to Buatsi, with the final judge a 95-95.
"It wasn't my finest showing but what defines champions? They win," the victor stated in the ringside as he was faced boos from the crowd.
The Londoner got back to winning after his maiden career setback - an thrilling contest with Callum Smith in February - to secure a twentieth professional win.
Several boxing experts believed the challenger, 31, merited the decision but the Derbyshire fighter endured the second defeat of his 28-match professional journey.
Buatsi Secures Triumph in Uneventful Encounter
Buatsi's progress has plateaued in recent years through contract problems and extended periods of inactivity.
Earlier forecast as the rising talent of UK boxing, he now finds himself trying to achieve expectations - maybe in what seems like a ultimate opportunity - under fresh management.
He began cautiously, with the two boxers prepared to take risks in a calculated initial rounds.
Parker, whose only previous defeat came at super middleweight against his former opponent in two years ago after suffering a hand injury mid-fight, was making just his second appearance at the higher weight class.
By the fight's midpoint, the underdog had kept it close, causing problems for Buatsi, who operated with a solid defense to narrowly win the cleaner periods.
The audience's tolerance decreased during constant clinching, with the challenger going down multiple occasions during grappling.
The underdog closed well, but multiple periods were hard to judge. Although, former world champion Carl Frampton - providing analysis for Dazn - had it significantly in favor for the challenger, scoring for Buatsi just only one stanza.
The victor advances, but with the likes of hard hitter David Benavidez, and Russian champion Dmitry Bivol leading the category, the journey to titles - on this evidence - still appears challenging.
Hatton Homage Inspires All-Manchester Contest
In the secondary fight, Lyndon Arthur beat Rea by close verdict in a outstanding local rivalry battle to secure the European light-heavyweight title.
The venue grew hushed for a profoundly moving tribute prior to the bout as a bell ceremony rang in remembrance of local hero Ricky Hatton, who succumbed 46 years old in September.
It was the premier televised boxing event in the urban center since the passing of Hatton. As the last bell disappeared, the quiet became a shout of "only one Ricky Hatton".
The competition during the bout matched the sentiment. The eventual winner was knocked down by a significant blow directly to the face in the early round, standing uncertainly to his footing on the referee's count.
But the red-colored attire of Arthur soon advanced again. Across the ring, Rea - in competitive blue - was marked up early, noticeable damage forming under his face as the duo landed hard punches.
The challenger, 27, landed power punches but the champion, thirty-four, fought fire with fire with stinging uppercuts as the intensity hardly diminished over the full distance.
The victor barely secured the decision with official scores of even score, 115-113 and wider gap, but both fighters were relentless and brave, exactly what their inspiration the legend stood for.
