UK light-heavyweight Joshua Buatsi earned a win yet disappointing debut to his career under fresh promotional backing with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's punch output seemingly secured him a split decision in a close ten-round contest at the Manchester venue.
But Parker landed significant shots, delivering effective single shots in a fight where the approaches never quite clicked, with frequent clinching spoiling the fight.
The scoring judges judged it 96-94 in favor to Buatsi, with the final judge a 95-95.
"It wasn't my greatest showing but what do champions do? They succeed," Buatsi commented in the ring as he was greeted with boos from the crowd.
The British fighter returned to winning ways after his first professional defeat - an epic battle with his previous opponent in last winter - to earn a twentieth professional win.
Various analysts thought the challenger, thirty-one, merited the decision but the contender experienced the second defeat of his 28-match professional journey.
Buatsi Earns Win in Slow Match
His career development has slowed in recent times through contract problems and extended periods of inactivity.
Once tipped as the rising talent of the British fight scene, he now finds himself trying to realize his potential - perhaps in what appears to be a last chance - under recent support.
He started carefully, with both competitors willing to overcommit in a calculated early stages.
Parker, whose single earlier setback came at the lower weight class against John Ryder in two years ago after damaging his hand mid-fight, was participating in his second appearance at light heavyweight.
By the middle rounds, Parker had made it competitive, causing problems for the favorite, who worked behind a protective stance to edge the better stanzas.
The audience's tolerance wore thin amid frequent holding, with Parker going down multiple occasions during clinches.
Parker finished the stronger, but many rounds were difficult to score. Although, former world champion the boxing analyst - commentating on Dazn - had it strongly leaning for Parker, giving Buatsi just a single round.
Buatsi moves on, but with the likes of heavy hitter David Benavidez, and Russian star Dmitry Bivol ruling the division, the journey to titles - based on this performance - still appears challenging.
Ricky Hatton Memorial Fuels All-Manchester Battle
In the secondary fight, Lyndon Arthur defeated Rea by close verdict in a outstanding local rivalry battle to win the European light-heavyweight championship.
The venue grew hushed for a profoundly moving memorial before the fight as a bell ceremony tolled in honor of the city's native Ricky Hatton, who died 46 years old in September.
It was the initial significant broadcast match card in the city since the passing of Hatton. As the last bell ended, the silence gave way to a shout of "only one Ricky Hatton".
The fighting within the ring matched the feeling. The eventual winner was floored by a powerful punch directly to the face in the second round, getting up unsteadily to his footing on the count of eight.
But the traditional red shorts of Arthur soon pressed ahead again. From the other corner, the opponent - in opposing blue - was marked up early, significant bruising developing under his face as the duo traded hard punches.
The challenger, 27, connected with strong blows but Arthur, thirty-four, matched the intensity with stinging uppercuts as the pace rarely dropped over twelve rounds.
The victor narrowly won the result with official scores of 114-114, close margin and clearer margin, but the two athletes were relentless and brave, exactly what their inspiration Hatton stood for.
