UK light heavyweight Joshua Buatsi earned a win yet underwhelming beginning to his career under recent management with a unanimous decision over Parker in Manchester.
The Olympic medal winner's activity level likely gained him a majority decision in a close ten-round contest at the Manchester venue.
But the challenger had his moments, landing some telling individual punches in a contest where the techniques never quite clicked, with constant grabbing marring the contest.
The scoring judges judged it 96-94 in favor to the winner, with the third a 95-95.
"It wasn't my best performance but what defines champions? They prevail," the victor commented in the ring as he was met by boos from the audience.
The British fighter bounced back after his maiden career setback - an memorable fight with Callum Smith in last winter - to secure a twentieth professional win.
Several boxing experts thought Parker, thirty-one, should have been awarded the win but the contender experienced the second loss of his 28-bout career.
The Champion Secures Win in Dull Affair
His career development has plateaued in recent years through promotional disputes and extended periods of inactivity.
Once tipped as the next star of the British fight scene, he now is positioned trying to fulfil that early promise - possibly in what seems like a last chance - under fresh management.
He started carefully, with neither fighter prepared to be aggressive in a calculated initial rounds.
Parker, whose only previous defeat came at super-middleweight against his former opponent in 2022 after breaking his hand in the contest, was making just his second appearance at light-heavyweight.
By the halfway stage, Parker had stayed in contention, making life awkward for the favorite, who worked behind a solid defense to edge the more effective rounds.
The audience's tolerance decreased amid constant clinching, with the challenger going down various instances during clinches.
Parker closed well, but many rounds were hard to judge. Nevertheless, ex-champion the boxing analyst - providing analysis for Dazn - had it particularly wide for the challenger, scoring for Buatsi just only one stanza.
Buatsi advances, but with the caliber of heavy hitter David Benavidez, and Russian star Dmitry Bivol ruling the division, the journey to titles - based on this performance - still seems difficult.
Hatton Homage Fuels Manchester Derby Contest
In the supporting bout, Lyndon Arthur overcame Rea by close verdict in a terrific Manchester vs Manchester contest to capture the European light-heavyweight belt.
The venue became quiet for a deeply poignant memorial ahead of the contest as ten bells sounded in memory of Manchester's own Hatton, who succumbed at age 46 in recently.
It was the premier televised boxing event in the urban center since the passing of Hatton. As the concluding sound faded, the quiet gave way to a chant of "there's only one Ricky Hatton".
The competition inside the ropes reflected the sentiment. The eventual winner was knocked down by a powerful punch flush on the face in the initial stage, getting up unsteadily to his feet on the eight count.
But the traditional red shorts of Arthur soon advanced again. From the other corner, Rea - in competitive blue - was injured early, significant bruising appearing under his eye as the fighters landed hard punches.
The challenger, twenty-seven, landed power punches but Arthur, 34, responded in kind with effective uppercuts as the pace seldom decreased over twelve rounds.
Arthur barely secured the result with official scores of draw, close margin and 115-112, but the two athletes were persistent and courageous, exactly what their inspiration the legend embodied.
