British light-heavyweight Joshua Buatsi secured a victory yet unimpressive start to life under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic bronze medallist's activity level seemingly secured him a split decision in a tight 10-round fight at the Manchester venue.
But Parker had his moments, connecting with powerful single shots in a contest where the approaches never quite clicked, with excessive holding spoiling the fight.
Two judges scored it 96-94 in favor to Buatsi, with the final judge a 95-95.
"It wasn't my greatest display but what do champions do? They succeed," the victor stated in the ringside as he was faced boos from the spectators.
The Londoner bounced back after his initial pro loss - an thrilling contest with Callum Smith in February - to earn a 20th career victory.
Several boxing experts believed the challenger, thirty-one, should have been awarded the decision but the Derbyshire fighter experienced the second defeat of his 28-match career.
Buatsi Secures Triumph in Slow Encounter
His career development has plateaued in recent times through promotional disputes and extended periods of inactivity.
Once tipped as the rising talent of UK boxing, he now is positioned trying to realize his potential - maybe in what seems like a last chance - under new promotional backing.
He began cautiously, with neither fighter willing to overcommit in a cagey opening few rounds.
The challenger, whose only previous defeat came at super-middleweight against his former opponent in two years ago after suffering a hand injury in the contest, was participating in his second bout at light-heavyweight.
By the halfway stage, Parker had made it competitive, creating difficulties for Buatsi, who utilized a solid defense to narrowly win the better stanzas.
The crowd's patience diminished because of frequent holding, with Parker going down various instances during close exchanges.
The underdog finished the stronger, but multiple periods were challenging to assess. Although, ex-champion the boxing analyst - commentating on the broadcast network - had it particularly wide for Parker, giving Buatsi just one round.
Buatsi moves on, but with the caliber of hard hitter David Benavidez, and Eastern European fighter Bivol ruling the division, the journey to titles - on this evidence - still appears challenging.
Ricky Hatton Memorial Inspires All-Manchester Battle
In the secondary fight, Arthur overcame Bradley Rea by close verdict in a terrific local rivalry fight to win the continental title.
The stadium grew hushed for a profoundly moving tribute ahead of the contest as a bell ceremony tolled in remembrance of Manchester's own Hatton, who succumbed at age 46 in September.
It was the first major televised match card in the location since the passing of Hatton. As the concluding sound disappeared, the quiet transitioned to a roar of "there's just one Hatton".
The competition within the ring equaled the feeling. Arthur was sent to the canvas by a significant blow squarely on the face in the early round, rising shakily to his feet on the count of eight.
But the Manchester United-red attire of Arthur soon came forward again. On the opposite side, Rea - in rival City blue - was marked up quickly, noticeable damage developing under his eye as the pair exchanged hard punches.
Rea, 27, connected with strong blows but Arthur, thirty-four, fought fire with fire with effective uppercuts as the pace hardly diminished over the full distance.
The victor narrowly won the result with official scores of 114-114, 115-113 and wider gap, but both fighters were persistent and courageous, everything their inspiration the legend embodied.
