UK light heavyweight Joshua Buatsi earned a win yet underwhelming start to his career under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic medal winner's work rate appeared to earn him a judges' verdict in a competitive 10-round fight at the Co-op Live Arena.
But Parker landed significant shots, delivering effective clean blows in a match where the approaches didn't complement each other, with constant grabbing marring the fight.
Two judges had it 96-94 in favor to Buatsi, with the final judge a 95-95 draw.
"It wasn't my greatest display but how do champions respond? They win," Buatsi commented in the ringside as he was faced disapproval from the audience.
The Londoner got back to winning after his first professional defeat - an epic battle with his previous opponent in last winter - to secure a twentieth professional win.
Various analysts felt the challenger, thirty-one, should have been awarded the decision but the Derbyshire fighter endured the second defeat of his 28-fight career.
The Champion Claims Victory in Uneventful Encounter
The fighter's advancement has plateaued in the past few years through promotional disputes and considerable time of inactivity.
Previously predicted as the future of UK boxing, he now is positioned trying to realize his potential - maybe in what seems like a final roll of the dice - under new promotional backing.
He started carefully, with both competitors willing to be aggressive in a tentative opening few rounds.
The challenger, whose sole prior loss came at super-middleweight against John Ryder in 2022 after damaging his hand during the bout, was making just his second fight at the higher weight class.
By the middle rounds, Parker had stayed in contention, creating difficulties for Buatsi, who operated with a solid defense to edge the better stanzas.
The spectators' acceptance diminished because of constant clinching, with Parker falling to the floor several times during clinches.
Parker finished the stronger, but multiple periods were challenging to assess. Nevertheless, previous titleholder the boxing analyst - commentating on the broadcast network - had it significantly in favor for the challenger, giving Buatsi just only one stanza.
The victor progresses, but with the likes of hard hitter Benavidez, and Russian champion Bivol leading the category, the road to world glory - based on this performance - still appears challenging.
Ricky Hatton Homage Fuels Manchester Derby Contest
In the supporting bout, Lyndon Arthur beat Rea by close verdict in a excellent local rivalry contest to win the European light heavyweight title.
The stadium fell silent for a deeply poignant memorial before the fight as ten bells tolled in remembrance of the city's native Hatton, who passed away 46 years old in September.
It was the first major televised match card in the location since Hatton's death. As the concluding sound faded, the quiet became a shout of "there's just one Hatton".
The competition within the ring equaled the sentiment. Arthur was sent to the canvas by a significant blow directly to the chin in the second round, getting up unsteadily to his feet on the referee's count.
But the Manchester United-red attire of Arthur soon pressed ahead again. Across the ring, Rea - in opposing blue - was marked up soon, significant bruising developing under his orbital area as the pair exchanged powerful shots.
Rea, 27, connected with strong blows but Arthur, 34, matched the intensity with effective uppercuts as the pace rarely dropped over twelve rounds.
The victor edged the decision with official scores of draw, 115-113 and wider gap, but both fighters were relentless and full of heart, all the qualities their inspiration the legend represented.
