UK light heavyweight Joshua Buatsi earned a win yet unimpressive debut to life under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic medal winner's work rate appeared to earn him a judges' verdict in a close 10-round fight at the Co-op Live Arena.
But Parker found success, connecting with powerful individual punches in a match where the techniques never quite clicked, with frequent clinching disrupting the contest.
The scoring judges had it 96-94 for to Buatsi, with the final judge a 95-95 draw.
"It wasn't my greatest display but how do champions respond? They succeed," Buatsi said in the ringside as he was faced disapproval from the crowd.
The Londoner bounced back after his maiden career setback - an thrilling contest with his previous opponent in February - to secure a twentieth professional win.
Various analysts thought the challenger, 31, deserved the victory but the contender suffered the second defeat of his 28-fight professional journey.
The Champion Earns Triumph in Uneventful Affair
Buatsi's progress has plateaued in recent times through promotional disputes and extended periods of inactivity.
Previously predicted as the future of UK boxing, he now is positioned trying to achieve expectations - maybe in what seems like a final roll of the dice - under new promotional backing.
He opened tentatively, with neither fighter ready to overcommit in a calculated opening few rounds.
Parker, whose sole prior loss 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 fight's midpoint, the underdog had made it competitive, making life awkward for the favorite, who utilized a protective stance to edge the better periods.
The crowd's patience decreased amid regular grabbing, with Parker falling to the floor several times during close exchanges.
Parker ended stronger, but many rounds were difficult to score. Nevertheless, previous titleholder Carl Frampton - commentating on Dazn - had it strongly leaning for Parker, scoring for Buatsi just a single round.
Buatsi advances, but with the caliber of hard hitter Benavidez, and Eastern European fighter Bivol dominating the weight class, the path to championship success - on this evidence - still looks steep.
Ricky Hatton Tribute Fuels Local Rivalry Fight
In the secondary fight, Arthur beat Bradley Rea by close verdict in a terrific Manchester vs Manchester fight to win the European light-heavyweight title.
The stadium fell silent for a deeply poignant memorial before the fight as 10 bells sounded in memory of Manchester's own Hatton, who died at age 46 in recently.
It was the initial significant broadcast boxing event in the location since Hatton's death. As the last bell faded, the silence transitioned to a shout of "only one Hatton".
The action during the bout reflected the feeling. Arthur was floored by a left hook directly to the chin in the initial stage, standing uncertainly to his feet on the count of eight.
But the red-colored shorts of the fighter soon came forward again. On the opposite side, the opponent - in rival City blue - was damaged soon, noticeable damage developing under his eye as the fighters exchanged powerful shots.
Rea, 27, landed power punches but Arthur, thirty-four, responded in kind with stinging uppercuts as the action seldom decreased over the full distance.
The victor barely secured the verdict with official scores of 114-114, close margin and wider gap, but both fighters were persistent and full of heart, exactly what their hero Hatton embodied.
