British light-heavyweight Joshua Buatsi secured a victory yet underwhelming debut to his career under new promoters with a points victory over Parker in Manchester.
The Olympic medal winner's work rate likely gained him a split decision in a competitive 10-round bout at the Manchester venue.
But Parker had his moments, delivering effective clean blows in a contest where the approaches didn't complement each other, with excessive holding marring the contest.
The officials had it 96-94 for to Buatsi, with the remaining official a even score.
"It wasn't my finest showing but what do champions do? They prevail," the victor commented in the ring as he was faced boos from the audience.
The British fighter returned to winning ways after his initial pro loss - an thrilling contest with Callum Smith in last winter - to secure a 20th career victory.
Several boxing experts thought the challenger, 31, should have been awarded the victory but the Derbyshire fighter experienced the second defeat of his 28-bout professional journey.
Buatsi Secures Victory in Slow Affair
The fighter's advancement has stalled in the past few years through contract problems and extended periods of inactivity.
Earlier forecast as the next star of UK boxing, he now finds himself trying to fulfil that early promise - possibly in what seems like a ultimate opportunity - under recent support.
He opened tentatively, with both competitors ready to be aggressive in a calculated early stages.
The challenger, whose single earlier setback came at super-middleweight against John Ryder in 2022 after breaking his hand during the bout, was competing in only his second appearance at light heavyweight.
By the halfway stage, the underdog had made it competitive, causing problems for Buatsi, who utilized a solid defense to edge the cleaner stanzas.
The crowd's patience wore thin because of regular grabbing, with Parker falling to the floor several times during clinches.
The underdog finished the stronger, but multiple periods were challenging to assess. Nevertheless, previous titleholder Carl Frampton - commentating on the broadcast network - had it particularly wide for Parker, awarding Buatsi just only one stanza.
Buatsi advances, but with the presence of heavy hitter David Benavidez, and Russian champion Dmitry Bivol ruling the division, the road to world glory - on this evidence - still looks steep.
Hatton Homage Fuels Local Rivalry Battle
In the secondary fight, Lyndon Arthur overcame Rea by divided decision in a outstanding local rivalry contest to capture the continental championship.
The venue became quiet for a deeply poignant homage ahead of the contest as ten bells sounded in remembrance of the city's native Hatton, who passed away aged 46 in recently.
It was the premier televised match card in the location since Hatton's passing. As the concluding sound faded, the stillness became a shout of "there's only one Hatton".
The fighting within the ring matched the sentiment. Arthur was sent to the canvas by a significant blow squarely on the jaw in the initial stage, rising shakily to his footing on the eight count.
But the Manchester United-red equipment of the fighter soon came forward again. Across the ring, Rea - in rival City blue - was marked up soon, significant bruising appearing under his eye as the pair exchanged punishing blows.
Rea, twenty-seven, landed power punches but the champion, 34, fought fire with fire with effective uppercuts as the intensity rarely dropped over the full distance.
The victor edged the result with scoring tallies of even score, narrow lead and clearer margin, but the two athletes were determined and brave, everything their hero the legend represented.
