British light-heavyweight Joshua Buatsi secured a victory yet unimpressive start to his career under recent management with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's activity level appeared to earn him a majority decision in a tight 10-round bout at the Manchester venue.
But the challenger landed significant shots, delivering effective individual punches in a fight where the styles didn't complement each other, with excessive holding marring the contest.
The officials scored it 96-94 to the winner, with the final judge a 95-95 draw.
"It wasn't my finest showing but how do champions respond? They prevail," Buatsi said in the ring as he was faced negative reactions from the spectators.
The Londoner bounced back after his first professional defeat - an memorable fight with Callum Smith in February - to earn a 20th career victory.
Several boxing experts felt the challenger, 31, should have been awarded the win but the Derbyshire fighter experienced the second loss of his 28-bout career.
Buatsi Earns Win in Slow Affair
His career development has stalled in recent times through management issues and considerable time of lack of fights.
Previously predicted as the future of the British fight scene, he now finds himself trying to achieve expectations - possibly in what feels like a final roll of the dice - under new promotional backing.
He opened tentatively, with neither fighter willing to be aggressive in a cagey early stages.
Parker, whose single earlier setback came at super-middleweight against John Ryder in 2022 after breaking his hand during the bout, was making just his second appearance at light heavyweight.
By the halfway stage, Parker had made it competitive, creating difficulties for the favorite, who operated with a protective stance to narrowly win the cleaner periods.
The crowd's patience diminished during constant clinching, with Parker going down several times during grappling.
Parker finished the stronger, but multiple periods were hard to judge. Nevertheless, previous titleholder Carl Frampton - commentating on Dazn - had it particularly wide for Parker, scoring for Buatsi just a single round.
Buatsi progresses, but with the presence of hard hitter Benavidez, and Russian champion Bivol ruling the division, the road to world glory - on this evidence - still looks steep.
Hatton Homage Motivates Manchester Derby Contest
In the co-main event, Lyndon Arthur beat Bradley Rea by split-decision in a excellent all-Manchester fight to win the European light-heavyweight title.
The stadium fell silent for a profoundly moving tribute before the fight as a bell ceremony rang in honor of Manchester's own Hatton, who died 46 years old in September.
It was the initial significant broadcast fight night in the urban center since Hatton's passing. As the concluding sound faded, the quiet became a chant of "there's just one Hatton".
The fighting inside the ropes reflected the sentiment. Arthur was floored by a powerful punch directly to the face in the second round, getting up unsteadily to his feet on the eight count.
But the traditional red equipment of Arthur soon advanced again. On the opposite side, the opponent - in rival City blue - was injured early, significant bruising forming under his eye as the pair traded hard punches.
Rea, twenty-seven, connected with strong blows but Arthur, thirty-four, fought fire with fire with effective uppercuts as the action hardly diminished over the full distance.
Arthur narrowly won the decision with official scores of draw, narrow lead and clearer margin, but each competitor were determined and full of heart, all the qualities their idol the legend represented.
