British light-heavyweight Joshua Buatsi earned a win yet unimpressive start to life under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic medal winner's punch output seemingly secured him a majority decision in a tight 10-round fight at the boxing arena.
But the challenger landed significant shots, connecting with powerful individual punches in a fight where the techniques didn't complement each other, with excessive holding disrupting the action.
The scoring judges had it 96-94 for to the winner, with the remaining official a 95-95 draw.
"It wasn't my best showing but what defines champions? They succeed," the victor stated in the ring as he was greeted with negative reactions from the audience.
The British fighter bounced back after his initial pro loss - an thrilling contest with Callum Smith in February - to earn a 20th pro success.
Some ringside observers believed Parker, thirty-one, should have been awarded the decision but the contender experienced the second setback of his 28-fight career.
The Champion Secures Triumph in Dull Affair
His career development has stalled in recent years through management issues and extended periods of inactivity.
Earlier forecast as the next star of UK boxing, he now is positioned trying to realize his potential - perhaps in what feels like a last chance - under new promotional backing.
He opened tentatively, with the two boxers ready to be aggressive in a tentative initial rounds.
Parker, whose sole prior loss came at super middleweight against John Ryder in two years ago after breaking his hand mid-fight, was making just his second appearance at light-heavyweight.
By the fight's midpoint, Parker had made it competitive, making life awkward for the favorite, who operated with a tight guard to barely take the cleaner stanzas.
The spectators' acceptance decreased amid frequent holding, with Parker going down multiple occasions during clinches.
The underdog closed well, but several stanzas were hard to judge. However, previous titleholder the boxing analyst - providing analysis for Dazn - had it significantly in favor for the challenger, scoring for Buatsi just only one stanza.
Buatsi progresses, but with the presence of heavy hitter David Benavidez, and Russian champion Dmitry Bivol leading the category, the road to world glory - based on this performance - still looks steep.
Ricky Hatton Homage Inspires All-Manchester Contest
In the co-main event, Lyndon Arthur overcame Rea by split-decision in a outstanding all-Manchester battle to capture the continental belt.
The stadium fell silent for a profoundly moving tribute before the fight as ten bells tolled in honor of Manchester's own Hatton, who passed away aged 46 in recently.
It was the initial significant broadcast fight night in the city since the passing of Hatton. As the concluding sound ended, the silence became a chant of "there's just one Hatton".
The fighting within the ring matched the feeling. Arthur was floored by a left hook directly to the jaw in the initial stage, standing uncertainly to his position on the count of eight.
But the Manchester United-red equipment of the fighter soon pressed ahead again. On the opposite side, Rea - in rival City blue - was damaged soon, heavy swelling developing under his orbital area as the duo exchanged punishing blows.
Rea, twenty-seven, delivered effective strikes but the champion, thirty-four, matched the intensity with effective uppercuts as the pace seldom decreased over the full distance.
The victor barely secured the decision with scoring tallies of draw, close margin and clearer margin, but both fighters were determined and full of heart, exactly what their inspiration Hatton embodied.
