UK light heavyweight Joshua Buatsi secured a victory yet unimpressive start to life under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic bronze medallist's punch output seemingly secured him a judges' verdict in a close 10-round bout at the Manchester venue.
But Parker landed significant shots, landing some telling individual punches in a match where the techniques never quite clicked, with constant grabbing spoiling the fight.
Two judges scored it 96-94 in favor to Buatsi, with the remaining official a even score.
"It wasn't my greatest display but what do champions do? They prevail," the victor stated in the ringside as he was greeted with disapproval from the spectators.
The British fighter bounced back after his maiden career setback - an epic battle with Callum Smith in February - to earn a twentieth professional win.
Various analysts felt the challenger, thirty-one, merited the win but the contender endured the second loss of his 28-match professional journey.
The Champion Secures Win in Dull Encounter
Buatsi's progress has stalled in recent years through promotional disputes and considerable time of inactivity.
Earlier forecast as the next star of British boxing, he now is positioned trying to achieve expectations - perhaps in what feels like a last chance - under fresh management.
He opened tentatively, with both competitors prepared to overcommit in a tentative 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 making just his second appearance at light-heavyweight.
By the halfway stage, the underdog had stayed in contention, making life awkward for Buatsi, who utilized a protective stance to barely take the better stanzas.
The spectators' acceptance decreased during frequent holding, with Parker going down several times during grappling.
Parker ended stronger, but many rounds were difficult to score. Although, previous titleholder the boxing analyst - providing analysis for Dazn - had it significantly in favor for the challenger, scoring for Buatsi just a single round.
The victor moves on, but with the caliber of power puncher Benavidez, and Eastern European fighter Bivol ruling the division, the road to world glory - judging by this showing - still looks steep.
Ricky Hatton Homage Motivates Local Rivalry Battle
In the co-main event, Arthur beat Rea by split-decision in a terrific local rivalry battle to win the European light heavyweight title.
The arena fell silent for a extremely emotional tribute ahead of the contest as a bell ceremony sounded in memory of the city's native Hatton, who succumbed aged 46 in September.
It was the initial significant broadcast match card in the city since the passing of Hatton. As the concluding sound faded, the silence gave way to a shout of "there's just one Hatton".
The fighting within the ring matched the sentiment. Arthur was sent to the canvas by a significant blow directly to the chin in the early round, getting up unsteadily to his footing on the count of eight.
But the red-colored attire of Arthur soon pressed ahead again. From the other corner, the opponent - in rival City blue - was damaged quickly, significant bruising forming under his face as the duo landed powerful shots.
The challenger, twenty-seven, connected with strong blows but the champion, 34, responded in kind with stinging uppercuts as the pace hardly diminished over twelve rounds.
The victor edged the result with scoring tallies of draw, 115-113 and wider gap, but each competitor were determined and full of heart, exactly what their inspiration Hatton represented.
