British light-heavyweight Joshua Buatsi made a winning yet disappointing beginning to life under recent management with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's work rate appeared to earn him a majority decision in a competitive ten-round bout at the Manchester venue.
But the challenger landed significant shots, landing some telling clean blows in a fight where the styles never quite clicked, with excessive holding disrupting the fight.
The officials had it 96-94 in favor to Buatsi, with the third a 95-95 draw.
"It wasn't my best performance but what defines champions? They succeed," Buatsi said in the arena as he was greeted with negative reactions from the crowd.
The Londoner got back to winning after his first professional defeat - an memorable fight with his previous opponent in last winter - to claim a 20th career victory.
Some ringside observers believed Parker, thirty-one, should have been awarded the victory but the contender endured the second defeat of his 28-fight professional journey.
Buatsi Earns Victory in Slow Match
His career development has plateaued in the past few years through promotional disputes and long spells of lack of fights.
Previously predicted as the rising talent of British boxing, he now finds himself trying to realize his potential - maybe in what feels like a last chance - under fresh management.
He began cautiously, with neither fighter willing to be aggressive in a tentative early stages.
The challenger, whose single earlier setback came at the lower weight class against his former opponent in two years ago after suffering a hand injury in the contest, was participating in his second appearance at light heavyweight.
By the halfway stage, Parker had stayed in contention, making life awkward for the favorite, who operated with a protective stance to edge the more effective stanzas.
The spectators' acceptance decreased amid frequent holding, with the challenger slipping to the canvas several times during grappling.
The underdog ended stronger, but many rounds were difficult to score. However, previous titleholder Carl Frampton - commentating on Dazn - had it strongly leaning for the challenger, giving Buatsi just only one stanza.
The victor advances, but with the likes of power puncher Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the path to championship success - judging by this showing - still appears challenging.
Ricky Hatton Tribute Motivates Local Rivalry Battle
In the supporting bout, Arthur defeated Rea by divided decision in a outstanding local rivalry contest to secure the continental belt.
The arena became quiet for a deeply poignant homage before the fight as 10 bells rang in honor of the city's native Ricky Hatton, who passed away aged 46 in September.
It was the first major televised boxing event in the urban center since Hatton's passing. As the last bell disappeared, the quiet gave way to a shout of "only one Ricky Hatton".
The action during the bout reflected the feeling. Arthur was knocked down by a powerful punch directly to the chin in the early round, standing uncertainly to his feet on the referee's count.
But the traditional red shorts of the fighter soon came forward again. From the other corner, the opponent - in opposing blue - was damaged early, noticeable damage developing under his eye as the fighters exchanged hard punches.
Rea, 27, landed power punches but Arthur, thirty-four, fought fire with fire with effective uppercuts as the intensity hardly diminished over 12 rounds.
The victor narrowly won the decision with official scores of even score, narrow lead and 115-112, but both fighters were relentless and courageous, exactly what their inspiration Hatton stood for.
