UK light heavyweight Joshua Buatsi secured a victory yet unimpressive start to life under recent management with a points victory over Parker in Manchester.
The Olympic medal winner's punch output appeared to earn him a judges' verdict in a competitive 10-round bout at the boxing arena.
But the challenger had his moments, landing some telling single shots in a match where the techniques never quite clicked, with frequent clinching spoiling the contest.
The scoring judges had it 96-94 in favor to Buatsi, with the remaining official a even score.
"It wasn't my best performance but how do champions respond? They succeed," the victor 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 February - to secure a twentieth professional win.
Some ringside observers felt Parker, 31, merited the win but the contender suffered the second setback of his 28-bout professional journey.
The Champion Secures Triumph in Dull Encounter
His career development has plateaued in the past few years through promotional disputes and considerable time of lack of fights.
Once tipped as the next star of the British fight scene, he now stands trying to realize his potential - perhaps in what appears to be a ultimate opportunity - under new promotional backing.
He started carefully, with the two boxers prepared to take risks in a tentative initial rounds.
The challenger, whose only previous defeat came at super middleweight against his former opponent in 2022 after suffering a hand injury during the bout, was competing in only his second fight at light-heavyweight.
By the fight's midpoint, the underdog had stayed in contention, making life awkward for the favorite, who utilized a solid defense to barely take the better stanzas.
The crowd's patience diminished during frequent holding, with the challenger going down various instances during grappling.
Parker ended stronger, but several stanzas were difficult to score. Although, ex-champion the boxing analyst - commentating on Dazn - had it significantly in favor for Parker, awarding Buatsi just one round.
The victor advances, but with the presence of hard hitter Benavidez, and Russian champion Bivol ruling the division, the journey to titles - judging by this showing - still appears challenging.
Hatton Tribute Inspires Manchester Derby Contest
In the supporting bout, Lyndon Arthur overcame Bradley Rea by close verdict in a excellent Manchester vs Manchester contest to win the continental championship.
The venue fell silent for a profoundly moving tribute prior to the bout as ten bells rang in remembrance of local hero Ricky Hatton, who succumbed at age 46 in September.
It was the initial significant broadcast match card in the city since Hatton's passing. As the final chime ended, the silence gave way to a shout of "only one Ricky Hatton".
The competition inside the ropes equaled the emotion. The eventual winner was knocked down by a significant blow squarely on the jaw in the second round, getting up unsteadily to his position on the eight count.
But the traditional red attire of the fighter soon came forward again. On the opposite side, the opponent - in rival City blue - was damaged early, heavy swelling forming under his orbital area as the pair traded punishing blows.
The challenger, 27, connected with strong blows but the champion, 34, matched the intensity with powerful upward punches as the pace hardly diminished over twelve rounds.
Arthur barely secured the decision with scoring tallies of even score, narrow lead and 115-112, but the two athletes were relentless and full of heart, everything their idol Hatton embodied.
