UK 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 work rate appeared to earn him a majority decision in a close ten-round contest at the boxing arena.
But the challenger landed significant shots, delivering effective clean blows in a match where the styles failed to mesh, with excessive holding spoiling the fight.
Two judges had it 96-94 to the winner, with the third a 95-95.
"It wasn't my finest display but what do champions do? They win," the victor said in the ring as he was faced negative reactions from the crowd.
The British fighter bounced back after his maiden career setback - an thrilling contest with Callum Smith in February - to claim a 20th career victory.
Various analysts believed the challenger, 31, should have been awarded the win but the Derbyshire fighter suffered the second defeat of his 28-bout professional journey.
Buatsi Claims Win in Uneventful Affair
The fighter's advancement has slowed in the past few years through contract problems and considerable time of lack of fights.
Earlier forecast as the rising talent of British boxing, he now finds himself trying to achieve expectations - perhaps in what appears to be a last chance - under fresh management.
He began cautiously, with neither fighter ready to overcommit in a calculated opening few rounds.
The challenger, whose single earlier setback came at the lower weight class against his former opponent in 2022 after damaging his hand during the bout, was making just his second appearance at light-heavyweight.
By the middle rounds, the underdog had stayed in contention, causing problems for the favorite, who operated with a protective stance to edge the better periods.
The crowd's patience wore thin because of constant clinching, with Parker going down multiple occasions during close exchanges.
The underdog ended stronger, but many rounds were difficult to score. Although, previous titleholder the boxing analyst - providing analysis for Dazn - had it particularly wide for the challenger, giving Buatsi just a single round.
The victor progresses, but with the caliber of heavy hitter David Benavidez, and Eastern European fighter Bivol ruling the division, the journey to titles - judging by this showing - still appears challenging.
Ricky Hatton Homage Inspires Manchester Derby Contest
In the co-main event, Arthur beat Bradley Rea by close verdict in a excellent all-Manchester battle to secure the continental title.
The venue became quiet for a profoundly moving homage prior to the bout as ten bells sounded in memory of Manchester's own Hatton, who died aged 46 in last month.
It was the initial significant broadcast fight night in the city since the passing of Hatton. As the final chime ended, the quiet became a chant of "there's only one Ricky Hatton".
The fighting during the bout reflected the feeling. Arthur was sent to the canvas by a significant blow squarely on the jaw in the early round, getting up unsteadily to his feet on the count of eight.
But the red-colored attire of Arthur soon pressed ahead again. From the other corner, Rea - in competitive blue - was damaged soon, significant bruising appearing under his face as the fighters landed punishing blows.
The challenger, twenty-seven, connected with strong blows but Arthur, thirty-four, responded in kind with powerful upward punches as the intensity hardly diminished over the full distance.
The victor narrowly won the decision with scoring tallies of 114-114, close margin and 115-112, but the two athletes were determined and full of heart, exactly what their idol the legend stood for.
