British light-heavyweight Joshua Buatsi earned a win yet disappointing start to life under recent management with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's work rate appeared to earn him a majority decision in a tight 10-round contest at the boxing arena.
But Parker landed significant shots, connecting with powerful single shots in a match where the approaches didn't complement each other, with excessive holding disrupting the fight.
The scoring judges had it 96-94 for to Buatsi, with the remaining official a 95-95.
"It wasn't my greatest performance but what do champions do? They win," the victor said in the arena as he was greeted with negative reactions from the audience.
The Londoner bounced back after his maiden career setback - an memorable fight with Callum Smith in February - to claim a 20th career victory.
Some ringside observers thought the challenger, thirty-one, merited the win but the contender endured the second setback of his 28-fight professional journey.
The Champion Claims Triumph in Slow Affair
Buatsi's progress has stalled in the past few years through promotional disputes and long spells of lack of fights.
Earlier forecast as the rising talent of the British fight scene, he now finds himself trying to fulfil that early promise - perhaps in what appears to be a final roll of the dice - under recent support.
He began cautiously, with neither fighter willing to take risks in a cagey opening few rounds.
Parker, whose only previous defeat came at super middleweight against John Ryder in two years ago after damaging his hand mid-fight, was making just his second appearance at the higher weight class.
By the middle rounds, Parker had stayed in contention, creating difficulties for Buatsi, who utilized a tight guard to narrowly win the more effective stanzas.
The audience's tolerance decreased because of frequent holding, with Parker slipping to the canvas several times during grappling.
The underdog finished the stronger, but several stanzas were difficult to score. Although, ex-champion the boxing analyst - providing analysis for Dazn - had it strongly leaning for Parker, awarding Buatsi just one round.
The victor advances, but with the likes of hard hitter Benavidez, and Eastern European fighter Bivol leading the category, the journey to titles - on this evidence - still looks steep.
Hatton Memorial Motivates Manchester Derby Battle
In the supporting bout, Lyndon Arthur overcame Rea by split-decision in a terrific all-Manchester contest to secure the European light-heavyweight title.
The venue fell silent for a deeply poignant tribute before the fight as a bell ceremony tolled in honor of Manchester's own Hatton, who succumbed 46 years old in last month.
It was the first major televised boxing event in the urban center since the passing of Hatton. As the final chime faded, the silence gave way to a shout of "there's only one Hatton".
The fighting within the ring equaled the feeling. Arthur was floored by a powerful punch directly to the chin in the initial stage, rising shakily to his footing on the count of eight.
But the red-colored attire of the fighter soon pressed ahead again. From the other corner, Rea - in rival City blue - was injured quickly, noticeable damage appearing under his face as the duo traded powerful shots.
Rea, twenty-seven, connected with strong blows but Arthur, thirty-four, responded in kind with powerful upward punches as the intensity hardly diminished over 12 rounds.
The victor narrowly won the decision with judges scorecards of 114-114, 115-113 and 115-112, but both fighters were determined and brave, everything their idol the legend embodied.
