UK light heavyweight Joshua Buatsi earned a win yet disappointing beginning to life under recent management with a decision win over Parker in the Manchester arena.
The Olympic medal winner's activity level appeared to earn him a majority decision in a competitive ten-round fight at the boxing arena.
But Parker had his moments, connecting with powerful individual punches in a contest where the styles never quite clicked, with frequent clinching spoiling the contest.
Two judges scored it 96-94 for to Buatsi, with the third a 95-95.
"It wasn't my greatest showing but what defines champions? They prevail," Buatsi said in the arena as he was met by disapproval from the audience.
The British fighter bounced back after his initial pro loss - an epic battle with Callum Smith in February - to claim a 20th pro success.
Various analysts believed Parker, thirty-one, merited the decision but the contender endured the second defeat of his 28-match career.
Buatsi Secures Win in Dull Affair
The fighter's advancement has stalled in recent years through promotional disputes and long spells of lack of fights.
Previously predicted as the rising talent of British boxing, he now is positioned trying to achieve expectations - maybe in what feels like a last chance - under recent support.
He started carefully, with the two boxers prepared to be aggressive in a cagey early stages.
The challenger, whose sole prior loss came at super-middleweight against John Ryder in two years ago after suffering a hand injury in the contest, was making just his second bout at light-heavyweight.
By the halfway stage, Parker had made it competitive, creating difficulties for Buatsi, who operated with a protective stance to edge the cleaner stanzas.
The spectators' acceptance wore thin because of regular grabbing, with Parker slipping to the canvas various instances during grappling.
The underdog ended stronger, but several stanzas were hard to judge. However, former world champion the boxing analyst - providing analysis for Dazn - had it strongly leaning for Parker, scoring for Buatsi just a single round.
Buatsi progresses, but with the caliber of power puncher David Benavidez, and Russian star Dmitry Bivol leading the category, the path to championship success - on this evidence - still looks steep.
Hatton Tribute Motivates Local Rivalry Battle
In the supporting bout, Lyndon Arthur defeated Rea by split-decision in a terrific all-Manchester contest to win the European light-heavyweight belt.
The venue fell silent for a profoundly moving memorial ahead of the contest as a bell ceremony rang in memory of the city's native Hatton, who died aged 46 in last month.
It was the initial significant broadcast boxing event in the city since the passing of Hatton. As the concluding sound ended, the quiet became a shout of "there's just one Hatton".
The fighting during the bout equaled the emotion. Arthur was floored by a left hook directly to the jaw in the initial stage, rising shakily to his footing on the eight count.
But the red-colored attire of Arthur soon pressed ahead again. On the opposite side, Rea - in competitive blue - was damaged quickly, significant bruising appearing under his face as the duo traded powerful shots.
The challenger, 27, landed power punches but Arthur, thirty-four, responded in kind with stinging uppercuts as the pace rarely dropped over 12 rounds.
The victor narrowly won the decision with scoring tallies of draw, close margin and 115-112, but the two athletes were relentless and full of heart, all the qualities their inspiration the legend represented.
