UK light-heavyweight Joshua Buatsi made a winning yet underwhelming start to life under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic medal winner's work rate seemingly secured him a judges' verdict in a competitive ten-round bout at the boxing arena.
But the challenger landed significant shots, delivering effective clean blows in a fight where the techniques didn't complement each other, with constant grabbing marring the action.
The scoring judges had it 96-94 to the winner, with the remaining official a even score.
"It wasn't my greatest performance but what defines champions? They prevail," Buatsi stated in the ringside as he was faced boos from the spectators.
The Londoner got back to winning after his initial pro loss - an memorable fight with Callum Smith in last winter - to earn a 20th pro success.
Some ringside observers believed Parker, thirty-one, deserved the win but the British boxer suffered the second defeat of his 28-fight professional journey.
The Champion Earns Triumph in Slow Encounter
The fighter's advancement has plateaued in recent years through promotional disputes and long spells of lack of fights.
Earlier forecast as the next star of British boxing, he now finds himself trying to achieve expectations - maybe in what seems like a final roll of the dice - under recent support.
He started carefully, with neither fighter willing to overcommit in a tentative opening few rounds.
Parker, whose sole prior loss came at super-middleweight against John Ryder in 2022 after damaging his hand in the contest, was competing in only his second fight at the higher weight class.
By the fight's midpoint, Parker had made it competitive, creating difficulties for the favorite, who utilized a tight guard to edge the more effective periods.
The crowd's patience diminished because of constant clinching, with Parker going down multiple occasions during close exchanges.
The underdog ended stronger, but several stanzas were difficult to score. Nevertheless, ex-champion the boxing analyst - providing analysis for the broadcast network - had it strongly leaning for Parker, awarding Buatsi just only one stanza.
The victor moves on, but with the presence of power puncher Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the road to world glory - judging by this showing - still appears challenging.
Ricky Hatton Tribute Fuels Local Rivalry Fight
In the co-main event, Lyndon Arthur defeated Bradley Rea by close verdict in a outstanding local rivalry fight to capture the European light-heavyweight belt.
The stadium became quiet for a deeply poignant tribute prior to the bout as a bell ceremony sounded in honor of the city's native Ricky Hatton, who died 46 years old in September.
It was the initial significant broadcast match card in the urban center since the passing of Hatton. As the final chime ended, the stillness transitioned to a roar of "there's just one Hatton".
The fighting during the bout reflected the emotion. Arthur was floored by a left hook flush on the jaw in the early round, getting up unsteadily to his feet on the referee's count.
But the traditional red attire of Arthur soon advanced again. From the other corner, the opponent - in competitive blue - was marked up early, significant bruising forming under his orbital area as the fighters traded powerful shots.
Rea, twenty-seven, landed power punches but the champion, 34, matched the intensity with powerful upward punches as the intensity hardly diminished over 12 rounds.
Arthur barely secured the decision with official scores of even score, close margin and wider gap, but each competitor were relentless and full of heart, all the qualities their inspiration Hatton stood for.
