British light heavyweight Joshua Buatsi made a winning yet disappointing beginning to life under recent management with a decision win over Parker in Manchester.
The Olympic bronze medallist's activity level appeared to earn him a split decision in a tight 10-round fight at the Co-op Live Arena.
But Parker found success, landing some telling individual punches in a match where the approaches never quite clicked, with excessive holding spoiling the action.
Two judges had it 96-94 to the winner, with the final judge a 95-95 draw.
"It wasn't my finest display but how do champions respond? They win," the victor said in the arena as he was met by disapproval from the audience.
The British fighter got back to winning after his first professional defeat - an memorable fight with Callum Smith in February - to claim a 20th pro success.
Various analysts felt the challenger, thirty-one, should have been awarded the decision but the British boxer endured the second loss of his 28-bout professional journey.
Buatsi Earns Triumph in Dull Affair
Buatsi's progress has slowed in recent years through contract problems and considerable time of lack of fights.
Previously predicted as the next star of the British fight scene, he now is positioned trying to achieve expectations - perhaps in what appears to be a last chance - under fresh management.
He began cautiously, with neither fighter willing to take risks in a cagey initial rounds.
The challenger, whose single earlier setback came at super middleweight against John Ryder in 2022 after damaging his hand during the bout, was participating in his second appearance at light-heavyweight.
By the middle rounds, Parker had kept it close, causing problems for the favorite, who worked behind a solid defense to barely take the better periods.
The spectators' acceptance decreased during constant clinching, with the challenger falling to the floor multiple occasions during close exchanges.
Parker ended stronger, but multiple periods were difficult to score. However, previous titleholder Carl Frampton - commentating on Dazn - had it significantly in favor for Parker, scoring for Buatsi just a single round.
Buatsi moves on, but with the presence of heavy hitter David Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the road to world glory - judging by this showing - still seems difficult.
Ricky Hatton Tribute Motivates Manchester Derby Battle
In the co-main event, Lyndon Arthur beat Rea by split-decision in a outstanding all-Manchester contest to win the European light heavyweight championship.
The venue became quiet for a profoundly moving tribute before the fight as a bell ceremony tolled in memory of the city's native Ricky Hatton, who died at age 46 in September.
It was the first major televised match card in the city since the passing of Hatton. As the last bell ended, the quiet gave way to a roar of "only one Ricky Hatton".
The competition within the ring equaled the emotion. The eventual winner was sent to the canvas by a significant blow flush on the chin in the second round, standing uncertainly to his footing on the count of eight.
But the traditional red equipment of the fighter soon came forward again. Across the ring, Rea - in competitive blue - was damaged quickly, significant bruising appearing under his eye as the pair landed punishing blows.
Rea, twenty-seven, delivered effective strikes but Arthur, thirty-four, responded in kind with powerful upward punches as the intensity hardly diminished over 12 rounds.
The victor edged the result with judges scorecards of draw, narrow lead and wider gap, but both fighters were relentless and courageous, everything their inspiration Hatton stood for.
