UK light-heavyweight Joshua Buatsi secured a victory yet underwhelming debut to his career under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic bronze medallist's work rate appeared to earn him a judges' verdict in a tight 10-round bout at the Co-op Live Arena.
But Parker found success, connecting with powerful clean blows in a contest where the techniques didn't complement each other, with constant grabbing spoiling the fight.
The scoring judges judged it 96-94 to the winner, with the third a even score.
"It wasn't my finest performance but what defines champions? They win," Buatsi stated in the ring as he was faced boos from the audience.
The Londoner returned to winning ways after his maiden career setback - an epic battle with his previous opponent in February - to earn a twentieth professional win.
Some ringside observers believed Parker, 31, deserved the decision but the British boxer experienced the second loss of his 28-bout career.
Buatsi Earns Win in Uneventful Affair
Buatsi's progress has plateaued in recent times through promotional disputes and long spells of inactivity.
Once tipped as the future of British boxing, he now stands trying to achieve expectations - perhaps in what seems like a ultimate opportunity - under new promotional backing.
He started carefully, with both competitors willing to take risks in a tentative opening few rounds.
The challenger, whose only previous defeat came at the lower weight class against his former opponent in two years ago after suffering a hand injury mid-fight, was making just his second fight at the higher weight class.
By the fight's midpoint, Parker had stayed in contention, creating difficulties for the favorite, who utilized a tight guard to edge the more effective rounds.
The crowd's patience diminished because of frequent holding, with Parker going down multiple occasions during clinches.
Parker closed well, but multiple periods were challenging to assess. However, ex-champion Carl Frampton - providing analysis for Dazn - had it strongly leaning for the challenger, scoring for Buatsi just a single round.
The victor progresses, but with the caliber of hard hitter Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the path to championship success - judging by this showing - still looks steep.
Ricky Hatton Homage Fuels All-Manchester Battle
In the supporting bout, Lyndon Arthur overcame Bradley Rea by split-decision in a outstanding all-Manchester fight to capture the European light heavyweight title.
The venue fell silent for a extremely emotional memorial ahead of the contest as a bell ceremony tolled in memory of Manchester's own Hatton, who passed away aged 46 in September.
It was the premier televised fight night in the location since Hatton's death. As the last bell faded, the silence gave way to a shout of "there's just one Ricky Hatton".
The competition within the ring reflected the feeling. Arthur was floored by a powerful punch directly to the chin in the initial stage, rising shakily to his position on the referee's count.
But the red-colored attire of Arthur soon pressed ahead again. Across the ring, the opponent - in opposing blue - was injured early, heavy swelling appearing under his eye as the fighters landed hard punches.
Rea, 27, delivered effective strikes but the champion, thirty-four, matched the intensity with stinging uppercuts as the action seldom decreased over 12 rounds.
The victor edged the decision with scoring tallies of draw, narrow lead and clearer margin, but the two athletes were determined and courageous, exactly what their inspiration the legend stood for.
