British light-heavyweight Joshua Buatsi earned a win yet disappointing start to life under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's activity level seemingly secured him a majority decision in a tight ten-round contest at the boxing arena.
But the challenger landed significant shots, delivering effective clean blows in a contest where the styles didn't complement each other, with excessive holding spoiling the fight.
Two judges had it 96-94 in favor to the winner, with the third a 95-95 draw.
"It wasn't my best display but what defines champions? They win," the victor said in the ring as he was faced negative reactions from the audience.
The Londoner bounced back after his initial pro loss - an epic battle with Callum Smith in last winter - to secure a 20th career victory.
Some ringside observers thought Parker, 31, should have been awarded the win but the contender suffered the second defeat of his 28-bout professional journey.
The Champion Claims Victory in Dull Encounter
His career development has stalled in the past few years through contract problems and extended periods of inactivity.
Once tipped as the next star of UK boxing, he now stands trying to achieve expectations - perhaps in what feels like a ultimate opportunity - under fresh management.
He began cautiously, with both competitors willing to take risks in a cagey initial rounds.
Parker, whose only previous defeat came at the lower weight class against John Ryder in 2022 after suffering a hand injury mid-fight, was participating in his second bout at the higher weight class.
By the fight's midpoint, the underdog had made it competitive, creating difficulties for the favorite, who operated with a solid defense to narrowly win the better stanzas.
The spectators' acceptance decreased because of frequent holding, with the challenger falling to the floor various instances during clinches.
Parker ended stronger, but several stanzas were difficult to score. Although, previous titleholder Carl Frampton - commentating on Dazn - had it particularly wide for Parker, scoring for Buatsi just only one stanza.
Buatsi moves on, but with the likes of power puncher Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the road to world glory - on this evidence - still looks steep.
Ricky Hatton Homage Inspires Local Rivalry Contest
In the supporting bout, Lyndon Arthur overcame Bradley Rea by split-decision in a outstanding Manchester vs Manchester battle to win the European light heavyweight title.
The arena fell silent for a extremely emotional tribute before the fight as 10 bells tolled in memory of local hero Ricky Hatton, who died at age 46 in September.
It was the initial significant broadcast boxing event in the city since the passing of Hatton. As the concluding sound disappeared, the quiet became a roar of "there's only one Hatton".
The competition inside the ropes matched the feeling. The eventual winner was floored by a left hook directly to the chin in the second round, standing uncertainly to his feet on the referee's count.
But the red-colored equipment of Arthur soon came forward again. Across the ring, the opponent - in rival City blue - was damaged early, significant bruising forming under his eye as the pair landed hard punches.
The challenger, twenty-seven, connected with strong blows but Arthur, 34, fought fire with fire with powerful upward punches as the action hardly diminished over the full distance.
Arthur narrowly won the decision with judges scorecards of even score, 115-113 and clearer margin, but both fighters were determined and courageous, all the qualities their inspiration the legend embodied.
