British light heavyweight Joshua Buatsi earned a win yet disappointing start to his career under new promoters with a points victory over Parker in Manchester.
The Olympic medal winner's work rate seemingly secured him a split decision in a tight ten-round bout at the boxing arena.
But Parker had his moments, connecting with powerful single shots in a contest where the styles failed to mesh, with frequent clinching disrupting the fight.
The scoring judges had it 96-94 in favor to Buatsi, with the final judge a 95-95 draw.
"It wasn't my greatest showing but how do champions respond? They succeed," Buatsi stated in the arena as he was greeted with negative reactions from the crowd.
The British fighter got back to winning after his initial pro loss - an epic battle with his previous opponent in February - to earn a twentieth professional win.
Various analysts felt the challenger, 31, merited the decision but the British boxer endured the second setback of his 28-fight professional journey.
Buatsi Earns Victory in Slow Encounter
Buatsi's progress has slowed in recent years through contract problems and extended periods of inactivity.
Earlier forecast as the rising talent of British boxing, he now stands trying to fulfil that early promise - possibly in what feels like a ultimate opportunity - under new promotional backing.
He began cautiously, with both competitors willing to take risks 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 mid-fight, was making just his second fight at light heavyweight.
By the middle rounds, Parker had made it competitive, making life awkward for Buatsi, who utilized a protective stance to narrowly win the more effective stanzas.
The spectators' acceptance wore thin because of constant clinching, with the challenger slipping to the canvas multiple occasions during close exchanges.
The underdog ended stronger, but several stanzas were difficult to score. Although, ex-champion the boxing analyst - providing analysis for the broadcast network - had it particularly wide for the challenger, awarding Buatsi just only one stanza.
Buatsi moves on, but with the presence of hard hitter David Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the path to championship success - judging by this showing - still seems difficult.
Ricky Hatton Homage Motivates Local Rivalry Battle
In the co-main event, Lyndon Arthur overcame Rea by close verdict in a terrific Manchester vs Manchester battle to capture the European light-heavyweight championship.
The stadium became quiet for a deeply poignant tribute ahead of the contest as 10 bells sounded in memory of Manchester's own Ricky Hatton, who passed away 46 years old in last month.
It was the premier televised match card in the city since Hatton's passing. As the last bell ended, the silence became a roar of "there's just one Ricky Hatton".
The action during the bout reflected the sentiment. The eventual winner was sent to the canvas by a powerful punch flush on the face in the second round, standing uncertainly to his feet on the referee's count.
But the Manchester United-red shorts of Arthur soon pressed ahead again. From the other corner, Rea - in rival City blue - was injured soon, heavy swelling forming under his face as the fighters landed hard punches.
Rea, 27, delivered effective strikes but the champion, 34, fought fire with fire with effective uppercuts as the intensity rarely dropped over 12 rounds.
Arthur narrowly won the result with official scores of 114-114, 115-113 and clearer margin, but each competitor were determined and brave, everything their idol the legend stood for.
