UK light-heavyweight Joshua Buatsi secured a victory yet disappointing beginning to life under new promoters with a decision win over Parker in Manchester.
The Olympic bronze medallist's work rate appeared to earn him a majority decision in a tight ten-round contest at the Manchester venue.
But Parker landed significant shots, delivering effective clean blows in a fight where the approaches failed to mesh, with excessive holding spoiling the contest.
The officials judged it 96-94 for to the winner, with the remaining official a 95-95.
"It wasn't my best display but how do champions respond? They win," Buatsi said in the ringside as he was faced disapproval from the audience.
The British fighter returned to winning ways after his first professional defeat - an memorable fight with Callum Smith in last winter - to secure a 20th career victory.
Some ringside observers thought Parker, 31, merited the decision but the contender experienced the second defeat of his 28-match career.
Buatsi Earns Victory in Dull Affair
His career development has slowed in recent times through promotional disputes and long spells of inactivity.
Earlier forecast as the next star of UK boxing, he now is positioned trying to achieve expectations - maybe in what appears to be a ultimate opportunity - under recent support.
He opened tentatively, with both competitors prepared to be aggressive in a cagey initial rounds.
Parker, whose only previous defeat came at super middleweight against his former opponent in 2022 after suffering a hand injury in the contest, was participating in his second bout at light-heavyweight.
By the fight's midpoint, the underdog had made it competitive, creating difficulties for Buatsi, who worked behind a protective stance to barely take the more effective rounds.
The audience's tolerance diminished amid frequent holding, with Parker going down various instances during grappling.
Parker finished the stronger, but several stanzas were challenging to assess. Nevertheless, ex-champion Carl Frampton - commentating on the broadcast network - had it strongly leaning for Parker, scoring for Buatsi just a single round.
The victor moves on, but with the caliber of power puncher Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the journey to titles - judging by this showing - still appears challenging.
Hatton Memorial Inspires All-Manchester Contest
In the co-main event, Arthur defeated Bradley Rea by close verdict in a outstanding local rivalry battle to win the European light-heavyweight title.
The stadium became quiet for a profoundly moving homage ahead of the contest as a bell ceremony tolled in remembrance of local hero Ricky Hatton, who died 46 years old in last month.
It was the premier televised match card in the city since Hatton's passing. As the final chime disappeared, the quiet gave way to a roar of "there's just one Hatton".
The fighting inside the ropes equaled the feeling. Arthur was floored by a significant blow flush on the chin in the second round, standing uncertainly to his feet on the eight count.
But the red-colored shorts of the fighter soon advanced again. Across the ring, Rea - in rival City blue - was damaged quickly, noticeable damage appearing under his eye as the fighters traded punishing blows.
The challenger, 27, landed power punches but the champion, 34, matched the intensity with effective uppercuts as the pace seldom decreased over the full distance.
The victor edged the result with official scores of 114-114, close margin and wider gap, but both fighters were persistent and full of heart, exactly what their idol Hatton embodied.
