British light heavyweight Joshua Buatsi earned a win yet unimpressive start to his career under new promoters with a points victory over Parker in Manchester.
The Olympic medal winner's work rate appeared to earn him a judges' verdict in a competitive 10-round bout at the boxing arena.
But the challenger landed significant shots, connecting with powerful clean blows in a fight where the styles never quite clicked, with excessive holding disrupting the action.
The officials had it 96-94 to the winner, with the third a even score.
"It wasn't my best showing but what do champions do? They prevail," the victor said in the ringside as he was greeted with negative reactions from the audience.
The British fighter bounced back after his maiden career setback - an memorable fight with Callum Smith in February - to secure a twentieth professional win.
Several boxing experts believed Parker, 31, deserved the victory but the British boxer suffered the second defeat of his 28-fight professional journey.
Buatsi Earns Win in Uneventful Affair
His career development has plateaued in recent times through contract problems and extended periods of lack of fights.
Once tipped as the next star of the British fight scene, he now finds himself trying to achieve expectations - perhaps in what appears to be a last chance - under recent support.
He began cautiously, with both competitors prepared to be aggressive in a calculated early stages.
The challenger, whose only previous defeat came at super middleweight against John Ryder in 2022 after damaging his hand mid-fight, was competing in only his second appearance at the higher weight class.
By the fight's midpoint, the underdog had stayed in contention, making life awkward for Buatsi, who worked behind a tight guard to edge the more effective stanzas.
The audience's tolerance decreased during frequent holding, with Parker slipping to the canvas several times during close exchanges.
Parker finished the stronger, but several stanzas were difficult to score. Although, previous titleholder the boxing analyst - working with the broadcast network - had it strongly leaning for the challenger, giving Buatsi just a single round.
Buatsi moves on, but with the caliber of heavy hitter Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the path to championship success - based on this performance - still appears challenging.
Ricky Hatton Homage Motivates Manchester Derby Fight
In the secondary fight, Lyndon Arthur beat Rea by split-decision in a excellent all-Manchester fight to capture the European light-heavyweight title.
The arena became quiet for a profoundly moving tribute before the fight as 10 bells rang in honor of local hero Ricky Hatton, who succumbed 46 years old in last month.
It was the initial significant broadcast match card in the location since Hatton's passing. As the final chime disappeared, the silence became a shout of "only one Hatton".
The action during the bout equaled the emotion. Arthur was sent to the canvas by a left hook flush on the jaw in the early round, rising shakily to his feet on the referee's count.
But the traditional red attire of the fighter soon advanced again. On the opposite side, Rea - in competitive blue - was damaged soon, significant bruising developing under his orbital area as the duo landed hard punches.
Rea, 27, connected with strong blows but the champion, thirty-four, matched the intensity with powerful upward punches as the pace hardly diminished over twelve rounds.
Arthur narrowly won the decision with official scores of even score, narrow lead and wider gap, but the two athletes were determined and full of heart, exactly what their hero the legend embodied.
