UK light-heavyweight Joshua Buatsi earned a win yet underwhelming start to life under fresh promotional backing with a points victory over Parker in the Manchester arena.
The Olympic medal winner's punch output likely gained him a split decision in a tight 10-round fight at the boxing arena.
But Parker landed significant shots, connecting with powerful clean blows in a contest where the approaches failed to mesh, with frequent clinching marring the contest.
Two judges scored it 96-94 in favor to the winner, with the third a 95-95 draw.
"It wasn't my greatest performance but what do champions do? They prevail," the victor commented in the ringside as he was met by disapproval from the crowd.
The Londoner bounced back after his first professional defeat - an memorable fight with his previous opponent in last winter - to secure a 20th pro success.
Various analysts thought the challenger, 31, merited the victory but the contender endured the second defeat of his 28-fight professional journey.
The Champion Claims Victory in Dull Affair
Buatsi's progress has slowed in recent years through contract problems and considerable time of lack of fights.
Once tipped as the next star of British boxing, he now stands trying to fulfil that early promise - maybe in what seems like a last chance - under fresh management.
He began cautiously, with both competitors willing to be aggressive in a calculated opening few rounds.
Parker, whose single earlier setback came at super middleweight against his former opponent in 2022 after damaging his hand mid-fight, was making just his second bout at the higher weight class.
By the middle rounds, the underdog had stayed in contention, causing problems for Buatsi, who worked behind a solid defense to barely take the cleaner stanzas.
The spectators' acceptance wore thin amid constant clinching, with Parker slipping to the canvas multiple occasions during close exchanges.
Parker finished the stronger, but several stanzas were difficult to score. Nevertheless, ex-champion Carl Frampton - commentating on the broadcast network - had it significantly in favor for the challenger, awarding Buatsi just one round.
The victor moves on, but with the caliber of power puncher David Benavidez, and Russian star Dmitry Bivol ruling the division, the road to world glory - based on this performance - still appears challenging.
Hatton Homage Motivates All-Manchester Battle
In the co-main event, Lyndon Arthur defeated Rea by split-decision in a outstanding Manchester vs Manchester battle to secure the European light-heavyweight belt.
The arena became quiet for a extremely emotional memorial prior to the bout as ten bells rang in honor of the city's native Ricky Hatton, who died 46 years old in recently.
It was the initial significant broadcast fight night in the city since the passing of Hatton. As the last bell faded, the stillness became a chant of "there's only one Hatton".
The competition inside the ropes matched the emotion. The eventual winner was sent to the canvas by a powerful punch squarely on the face in the initial stage, getting up unsteadily to his feet on the count of eight.
But the red-colored shorts of Arthur soon pressed ahead again. On the opposite side, Rea - in rival City blue - was marked up soon, heavy swelling forming under his orbital area as the fighters landed powerful shots.
Rea, twenty-seven, connected with strong blows but Arthur, thirty-four, matched the intensity with effective uppercuts as the intensity hardly diminished over the full distance.
Arthur edged the decision with judges scorecards of 114-114, 115-113 and wider gap, but the two athletes were determined and brave, everything their inspiration Hatton represented.
