UK light-heavyweight Joshua Buatsi made a winning yet disappointing start to life under new promoters with a unanimous decision over Parker in Manchester.
The Olympic medal winner's work rate appeared to earn him a majority decision in a competitive ten-round fight at the Manchester venue.
But the challenger found success, delivering effective single shots in a fight where the approaches never quite clicked, with constant grabbing disrupting the action.
The scoring judges scored it 96-94 to Buatsi, with the remaining official a 95-95.
"It wasn't my best performance but what do champions do? They succeed," Buatsi said in the ring as he was greeted with negative reactions from the crowd.
The British fighter got back to winning after his maiden career setback - an epic battle with his previous opponent in last winter - to claim a 20th pro success.
Several boxing experts felt Parker, 31, deserved the win but the British boxer endured the second defeat of his 28-fight professional journey.
Buatsi Secures Triumph in Uneventful Encounter
The fighter's advancement has plateaued in recent times through management issues and considerable time of inactivity.
Once tipped as the future of British boxing, he now stands trying to achieve expectations - perhaps in what appears to be a ultimate opportunity - under fresh management.
He began cautiously, with the two boxers ready to be aggressive in a tentative early stages.
Parker, whose single earlier setback came at super middleweight against John Ryder in two years ago after suffering a hand injury mid-fight, was competing in only his second appearance at the higher weight class.
By the halfway stage, the underdog had kept it close, creating difficulties for Buatsi, who operated with a solid defense to narrowly win the better periods.
The audience's tolerance decreased amid frequent holding, with Parker slipping to the canvas various instances during clinches.
The underdog finished the stronger, but several stanzas were hard to judge. Nevertheless, ex-champion Carl Frampton - providing analysis for the broadcast network - had it strongly leaning for Parker, scoring for Buatsi just a single round.
The victor moves on, but with the likes of hard hitter David Benavidez, and Russian star Dmitry Bivol ruling the division, the road to world glory - based on this performance - still looks steep.
Hatton Homage Inspires All-Manchester Battle
In the secondary fight, Lyndon Arthur beat Rea by divided decision in a excellent local rivalry fight to win the European light heavyweight belt.
The venue fell silent for a deeply poignant tribute before the fight as 10 bells sounded in remembrance of local hero Hatton, who passed away aged 46 in September.
It was the premier televised fight night in the urban center since Hatton's death. As the concluding sound faded, the quiet gave way to a shout of "only one Hatton".
The fighting during the bout matched the feeling. The eventual winner was knocked down by a significant blow squarely on the jaw in the second round, rising shakily to his feet on the eight count.
But the red-colored attire of the fighter soon came forward again. On the opposite side, Rea - in rival City blue - was marked up quickly, significant bruising developing under his eye as the pair landed punishing blows.
Rea, twenty-seven, delivered effective strikes but Arthur, thirty-four, responded in kind with stinging uppercuts as the pace seldom decreased over 12 rounds.
The victor narrowly won the decision with judges scorecards of even score, narrow lead and wider gap, but both fighters were determined and courageous, all the qualities their inspiration the legend embodied.
