UK light heavyweight Joshua Buatsi earned a win yet underwhelming debut to his career under fresh promotional backing with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's work rate seemingly secured him a split decision in a close 10-round fight at the Manchester venue.
But the challenger had his moments, connecting with powerful individual punches in a fight where the techniques never quite clicked, with constant grabbing disrupting the action.
The officials judged it 96-94 for to the winner, with the remaining official a even score.
"It wasn't my finest performance but what do champions do? They prevail," Buatsi commented in the arena as he was greeted with negative reactions from the crowd.
The Londoner got back to winning after his maiden career setback - an memorable fight with Callum Smith in last winter - to claim a 20th career victory.
Several boxing experts thought the challenger, thirty-one, deserved the win but the British boxer suffered the second defeat of his 28-fight career.
The Champion Secures Win in Uneventful Encounter
The fighter's advancement has stalled in the past few years through contract problems and long spells of inactivity.
Previously predicted as the rising talent of UK boxing, he now is positioned trying to achieve expectations - perhaps in what feels like a last chance - under recent support.
He started carefully, with both competitors willing to be aggressive in a calculated early stages.
Parker, whose sole prior loss came at super middleweight against his former opponent in 2022 after suffering a hand injury in the contest, was competing in only his second bout at the higher weight class.
By the middle rounds, the underdog had kept it close, creating difficulties for the favorite, who utilized a protective stance to narrowly win the better rounds.
The spectators' acceptance wore thin because of regular grabbing, with Parker slipping to the canvas multiple occasions during close exchanges.
The underdog finished the stronger, but multiple periods were challenging to assess. Nevertheless, former world champion Carl Frampton - providing analysis for Dazn - had it strongly leaning for the challenger, giving Buatsi just only one stanza.
The victor advances, but with the likes of power puncher David Benavidez, and Russian champion Dmitry Bivol ruling the division, the journey to titles - judging by this showing - still seems difficult.
Ricky Hatton Tribute Motivates All-Manchester Contest
In the supporting bout, Lyndon Arthur defeated Bradley Rea by split-decision in a outstanding local rivalry fight to secure the European light heavyweight title.
The arena grew hushed for a profoundly moving homage before the fight as ten bells rang in remembrance of Manchester's own Ricky Hatton, who succumbed at age 46 in last month.
It was the first major televised fight night in the urban center since Hatton's death. As the last bell disappeared, the stillness became a chant of "there's only one Ricky Hatton".
The fighting within the ring matched the emotion. The eventual winner was knocked down by a significant blow squarely on the chin in the initial stage, getting up unsteadily to his position on the referee's count.
But the Manchester United-red shorts of the fighter soon came forward again. From the other corner, Rea - in opposing blue - was damaged early, heavy swelling appearing under his orbital area as the pair exchanged hard punches.
Rea, twenty-seven, landed power punches but the champion, 34, responded in kind with powerful upward punches as the pace hardly diminished over the full distance.
The victor barely secured the verdict with scoring tallies of even score, 115-113 and clearer margin, but each competitor were determined and full of heart, exactly what their hero Hatton represented.
