British light-heavyweight Joshua Buatsi earned a win yet underwhelming beginning to his career under recent management with a unanimous decision over Parker in Manchester.
The Olympic medal winner's work rate likely gained him a majority decision in a tight 10-round fight at the Manchester venue.
But the challenger landed significant shots, connecting with powerful clean blows in a fight where the techniques didn't complement each other, with frequent clinching marring the fight.
The officials scored it 96-94 for to Buatsi, with the final judge a 95-95.
"It wasn't my best showing but what do champions do? They win," the victor commented in the ringside as he was faced boos from the crowd.
The Londoner got back to winning after his initial pro loss - an epic battle with his previous opponent in last winter - to earn a twentieth professional win.
Various analysts believed Parker, 31, merited the victory but the contender suffered the second loss of his 28-fight career.
The Champion Claims Victory in Slow Affair
Buatsi's progress has stalled in the past few years through promotional disputes and considerable time of inactivity.
Earlier forecast as the future of the British fight scene, he now stands trying to realize his potential - maybe in what feels like a final roll of the dice - under new promotional backing.
He opened tentatively, with both competitors willing to be aggressive in a calculated initial rounds.
The challenger, whose sole prior loss came at super-middleweight against his former opponent in 2022 after suffering a hand injury in the contest, was making just his second bout at light-heavyweight.
By the fight's midpoint, Parker had stayed in contention, making life awkward for the favorite, who utilized a solid defense to edge the better stanzas.
The spectators' acceptance decreased amid regular grabbing, with the challenger going down several times during clinches.
Parker closed well, but several stanzas were hard to judge. However, ex-champion the boxing analyst - working with Dazn - had it particularly wide for the challenger, giving Buatsi just only one stanza.
The victor advances, but with the presence of heavy hitter Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the path to championship success - based on this performance - still looks steep.
Ricky Hatton Homage Inspires All-Manchester Contest
In the supporting bout, Lyndon Arthur defeated Bradley Rea by split-decision in a excellent local rivalry contest to secure the European light-heavyweight belt.
The venue grew hushed for a profoundly moving tribute ahead of the contest as a bell ceremony rang in remembrance of Manchester's own Hatton, who passed away aged 46 in recently.
It was the first major televised fight night in the city since Hatton's death. As the concluding sound ended, the quiet transitioned to a shout of "there's only one Ricky Hatton".
The competition during the bout equaled the feeling. Arthur was sent to the canvas by a significant blow directly to the jaw in the initial stage, getting up unsteadily to his feet on the count of eight.
But the traditional red shorts of Arthur soon pressed ahead again. On the opposite side, Rea - in competitive blue - was injured soon, heavy swelling forming under his eye as the pair exchanged hard punches.
Rea, 27, delivered effective strikes but Arthur, thirty-four, matched the intensity with powerful upward punches as the action rarely dropped over 12 rounds.
The victor edged the decision with official scores of draw, close margin and clearer margin, but each competitor were determined and full of heart, all the qualities their hero Hatton represented.
