British light-heavyweight Joshua Buatsi secured a victory yet underwhelming beginning to life under recent management with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's work rate seemingly secured him a judges' verdict in a competitive 10-round fight at the Manchester venue.
But the challenger landed significant shots, connecting with powerful single shots in a contest where the techniques never quite clicked, with excessive holding disrupting the fight.
Two judges had it 96-94 to Buatsi, with the third a even score.
"It wasn't my greatest showing but how do champions respond? They win," Buatsi stated in the arena as he was greeted with disapproval from the spectators.
The British fighter bounced back after his maiden career setback - an memorable fight with his previous opponent in February - to claim a twentieth professional win.
Some ringside observers thought Parker, 31, merited the win but the British boxer experienced the second defeat of his 28-fight career.
The Champion Claims Win in Uneventful Match
Buatsi's progress has stalled in recent years through promotional disputes and extended periods of inactivity.
Once tipped as the rising talent of the British fight scene, he now is positioned trying to achieve expectations - maybe in what appears to be a final roll of the dice - under recent support.
He began cautiously, with both competitors prepared to overcommit in a cagey initial rounds.
Parker, whose only previous defeat came at the lower weight class against his former opponent in two years ago after breaking his hand in the contest, was making just his second bout at light heavyweight.
By the fight's midpoint, the underdog had stayed in contention, causing problems for Buatsi, who utilized a tight guard to barely take the more effective periods.
The crowd's patience diminished amid regular grabbing, with the challenger falling to the floor several times during close exchanges.
Parker closed well, but multiple periods were hard to judge. Although, ex-champion Carl Frampton - commentating on Dazn - had it particularly wide for the challenger, awarding Buatsi just a single round.
The victor advances, but with the caliber of hard hitter Benavidez, and Russian champion Bivol ruling the division, the journey to titles - on this evidence - still appears challenging.
Ricky Hatton Homage Motivates All-Manchester Fight
In the secondary fight, Lyndon Arthur defeated Bradley Rea by divided decision in a outstanding Manchester vs Manchester fight to win the European light heavyweight belt.
The stadium fell silent for a deeply poignant memorial before the fight as a bell ceremony sounded in memory of local hero Hatton, who passed away aged 46 in September.
It was the first major televised fight night in the city since Hatton's passing. As the last bell disappeared, the silence became a roar of "there's only one Ricky Hatton".
The competition inside the ropes reflected the emotion. The eventual winner was sent to the canvas by a left hook squarely on the chin in the initial stage, standing uncertainly to his position on the eight count.
But the Manchester United-red attire of Arthur soon pressed ahead again. From the other corner, the opponent - in rival City blue - was marked up soon, significant bruising forming under his face as the pair exchanged punishing blows.
Rea, twenty-seven, connected with strong blows but Arthur, 34, matched the intensity with stinging uppercuts as the pace hardly diminished over 12 rounds.
Arthur barely secured the result with official scores of even score, 115-113 and clearer margin, but the two athletes were determined and courageous, everything their idol the legend stood for.
