UK light heavyweight Joshua Buatsi secured a victory yet disappointing start to life under new promoters with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's activity level appeared to earn him a majority decision in a close ten-round fight at the Manchester venue.
But the challenger landed significant shots, delivering effective individual punches in a contest where the styles never quite clicked, with constant grabbing marring the contest.
The officials judged it 96-94 for to the winner, with the third a 95-95 draw.
"It wasn't my greatest showing but what defines champions? They succeed," the victor commented in the ring as he was met by negative reactions from the crowd.
The British fighter bounced back after his maiden career setback - an thrilling contest with his previous opponent in last winter - to claim a 20th pro success.
Some ringside observers thought the challenger, 31, merited the decision but the Derbyshire fighter endured the second setback of his 28-match career.
The Champion Claims Victory in Slow Affair
The fighter's advancement has slowed in the past few years through management issues and long spells of inactivity.
Earlier forecast as the rising talent of British boxing, he now is positioned trying to achieve expectations - possibly in what appears to be a ultimate opportunity - under fresh management.
He began cautiously, with both competitors prepared to be aggressive in a tentative early stages.
Parker, whose single earlier setback came at super middleweight against John Ryder in 2022 after damaging his hand mid-fight, was competing in only his second bout at light heavyweight.
By the middle rounds, Parker had kept it close, making life awkward for the favorite, who worked behind a protective stance to edge the cleaner periods.
The crowd's patience wore thin amid frequent holding, with the challenger slipping to the canvas several times during grappling.
The underdog ended stronger, but several stanzas were challenging to assess. However, ex-champion the boxing analyst - providing analysis for the broadcast network - had it significantly in favor for Parker, scoring for Buatsi just a single round.
Buatsi progresses, but with the caliber of power puncher Benavidez, and Russian champion Dmitry Bivol ruling the division, the road to world glory - on this evidence - still looks steep.
Hatton Homage Motivates Local Rivalry Contest
In the co-main event, Arthur overcame Bradley Rea by divided decision in a outstanding local rivalry contest to win the European light-heavyweight title.
The stadium grew hushed for a extremely emotional homage prior to the bout as 10 bells rang in memory of the city's native Ricky Hatton, who succumbed aged 46 in September.
It was the initial significant broadcast match card in the city since Hatton's death. As the final chime ended, the stillness became a shout of "there's just one Hatton".
The action during the bout equaled the sentiment. Arthur was sent to the canvas by a left hook squarely on the jaw in the initial stage, rising shakily to his position on the referee's count.
But the traditional red attire of Arthur soon pressed ahead again. On the opposite side, Rea - in competitive blue - was injured soon, heavy swelling developing under his orbital area as the fighters traded hard punches.
Rea, twenty-seven, connected with strong blows but Arthur, 34, matched the intensity with powerful upward punches as the action hardly diminished over twelve rounds.
Arthur edged the result with judges scorecards of draw, narrow lead and clearer margin, but both fighters were determined and brave, all the qualities their inspiration the legend represented.
