UK light-heavyweight Joshua Buatsi earned a win yet disappointing start to life under new promoters with a decision win over Parker in Manchester.
The Olympic medal winner's activity level seemingly secured him a judges' verdict in a tight ten-round bout at the Manchester venue.
But Parker landed significant shots, connecting with powerful single shots in a fight where the styles failed to mesh, with frequent clinching disrupting the fight.
The scoring judges scored it 96-94 in favor to the winner, with the final judge a 95-95 draw.
"It wasn't my greatest display but what defines champions? They succeed," the victor said in the ring as he was greeted with boos from the crowd.
The British fighter bounced back after his maiden career setback - an epic battle with his previous opponent in February - to earn a 20th pro success.
Several boxing experts felt the challenger, 31, deserved the win but the Derbyshire fighter endured the second loss of his 28-match professional journey.
The Champion Secures Triumph in Dull Match
The fighter's advancement has plateaued in the past few years through management issues and considerable time of inactivity.
Earlier forecast as the rising talent of the British fight scene, he now finds himself trying to realize his potential - maybe in what appears to be a final roll of the dice - under fresh management.
He opened tentatively, with neither fighter willing to take risks in a cagey opening few rounds.
The challenger, whose sole prior loss came at super middleweight against John Ryder in two years ago after breaking his hand mid-fight, was participating in his second appearance at light heavyweight.
By the middle rounds, the underdog had stayed in contention, causing problems for the favorite, who worked behind a protective stance to narrowly win the better rounds.
The crowd's patience diminished because of constant clinching, with Parker slipping to the canvas multiple occasions during clinches.
The underdog closed well, but multiple periods were hard to judge. However, previous titleholder the boxing analyst - providing analysis for Dazn - had it significantly in favor for Parker, scoring for Buatsi just one round.
Buatsi moves on, but with the caliber of power puncher David Benavidez, and Russian champion Dmitry Bivol ruling the division, the path to championship success - based on this performance - still looks steep.
Ricky Hatton Tribute Inspires Manchester Derby Contest
In the supporting bout, Lyndon Arthur beat Bradley Rea by close verdict in a outstanding local rivalry fight to capture the continental title.
The venue grew hushed for a extremely emotional homage ahead of the contest as ten bells rang in honor of local hero Hatton, who passed away 46 years old in September.
It was the initial significant broadcast fight night in the urban center since Hatton's death. As the last bell faded, the quiet gave way to a shout of "only one Hatton".
The competition during the bout reflected the feeling. Arthur was floored by a significant blow directly to the jaw in the second round, getting up unsteadily to his position on the referee's count.
But the Manchester United-red attire of the fighter soon advanced again. On the opposite side, Rea - in rival City blue - was marked up soon, significant bruising appearing under his orbital area as the pair landed punishing blows.
The challenger, 27, connected with strong blows but the champion, thirty-four, responded in kind with stinging uppercuts as the intensity seldom decreased over the full distance.
Arthur barely secured the result with scoring tallies of draw, narrow lead and wider gap, but the two athletes were relentless and courageous, exactly what their hero the legend represented.
