UK light heavyweight Joshua Buatsi secured a victory yet unimpressive beginning to life under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's activity level likely gained him a majority decision in a competitive ten-round bout at the Co-op Live Arena.
But the challenger landed significant shots, landing some telling single shots in a fight where the approaches failed to mesh, with frequent clinching spoiling the action.
The scoring judges had it 96-94 in favor to the winner, with the third a 95-95.
"It wasn't my finest display but what defines champions? They prevail," Buatsi stated in the ringside as he was faced negative reactions from the audience.
The British fighter bounced back after his first professional defeat - an thrilling contest with his previous opponent in last winter - to secure a 20th pro success.
Some ringside observers believed Parker, thirty-one, deserved the win but the British boxer endured the second loss of his 28-fight career.
The Champion Claims Triumph in Dull Encounter
The fighter's advancement has slowed in recent times through management issues and extended periods of inactivity.
Previously predicted as the future of British boxing, he now is positioned trying to fulfil that early promise - maybe in what appears to be a final roll of the dice - under recent support.
He began cautiously, with neither fighter willing to overcommit in a calculated initial rounds.
Parker, whose sole prior loss came at the lower weight class against John Ryder in 2022 after suffering a hand injury mid-fight, was competing in only his second fight at light-heavyweight.
By the fight's midpoint, the underdog had stayed in contention, creating difficulties for Buatsi, who operated with a solid defense to edge the cleaner rounds.
The crowd's patience wore thin during regular grabbing, with Parker falling to the floor various instances during close exchanges.
The underdog ended stronger, but many rounds were difficult to score. However, previous titleholder the boxing analyst - providing analysis for the broadcast network - had it strongly leaning for the challenger, giving Buatsi just only one stanza.
Buatsi moves on, but with the presence of power puncher Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the road to world glory - judging by this showing - still seems difficult.
Hatton Memorial Fuels Local Rivalry Contest
In the supporting bout, Lyndon Arthur overcame Rea by split-decision in a outstanding Manchester vs Manchester battle to capture the European light-heavyweight title.
The arena fell silent for a profoundly moving memorial prior to the bout as a bell ceremony sounded in honor of Manchester's own Ricky Hatton, who died 46 years old in last month.
It was the premier televised fight night in the urban center since Hatton's passing. As the last bell faded, the stillness transitioned to a roar of "only one Ricky Hatton".
The action during the bout equaled the sentiment. Arthur was sent to the canvas by a significant blow directly to the chin in the early round, rising shakily to his feet on the eight count.
But the traditional red attire of Arthur soon came forward again. On the opposite side, the opponent - in competitive blue - was damaged soon, noticeable damage forming under his face as the duo exchanged powerful shots.
The challenger, twenty-seven, connected with strong blows but Arthur, 34, responded in kind with powerful upward punches as the pace rarely dropped over the full distance.
The victor edged the decision with judges scorecards of 114-114, narrow lead and 115-112, but both fighters were persistent and full of heart, all the qualities their inspiration the legend represented.
