UK light heavyweight Joshua Buatsi secured a victory yet unimpressive debut to life under new promoters with a decision win over Parker in Manchester.
The Olympic medal winner's punch output likely gained him a majority decision in a tight 10-round bout at the Co-op Live Arena.
But Parker landed significant shots, landing some telling single shots in a match where the styles didn't complement each other, with frequent clinching disrupting the fight.
The officials judged it 96-94 for to Buatsi, with the remaining official a 95-95 draw.
"It wasn't my best showing but what defines champions? They succeed," the victor commented in the ring as he was met by negative reactions from the audience.
The British fighter bounced back after his maiden career setback - an memorable fight with his previous opponent in last winter - to claim a 20th career victory.
Some ringside observers believed the challenger, thirty-one, merited the victory but the contender endured the second setback of his 28-match career.
The Champion Secures Win in Uneventful Encounter
Buatsi's progress has stalled in recent times through contract problems and long spells of inactivity.
Earlier forecast as the future of the British fight scene, he now stands trying to achieve expectations - perhaps in what seems like a last chance - under fresh management.
He opened tentatively, with both competitors willing to be aggressive in a tentative initial rounds.
Parker, whose sole prior loss came at super-middleweight against John Ryder in 2022 after damaging his hand during the bout, was participating in his second bout at the higher weight class.
By the middle rounds, Parker had kept it close, creating difficulties for Buatsi, who utilized a protective stance to edge the cleaner stanzas.
The spectators' acceptance diminished amid constant clinching, with the challenger slipping to the canvas various instances during grappling.
The underdog finished the stronger, but several stanzas were hard to judge. Although, previous titleholder Carl Frampton - working with the broadcast network - had it significantly in favor for the challenger, giving Buatsi just one round.
Buatsi progresses, but with the presence of heavy hitter David Benavidez, and Russian champion Dmitry Bivol ruling the division, the road to world glory - judging by this showing - still appears challenging.
Hatton Tribute Motivates All-Manchester Battle
In the supporting bout, Lyndon Arthur defeated Rea by close verdict in a outstanding Manchester vs Manchester fight to win the European light-heavyweight championship.
The stadium fell silent for a profoundly moving memorial ahead of the contest as a bell ceremony tolled in honor of Manchester's own Hatton, who passed away aged 46 in recently.
It was the initial significant broadcast boxing event in the city since the passing of Hatton. As the last bell disappeared, the quiet transitioned to a chant of "there's just one Hatton".
The competition inside the ropes equaled the sentiment. The eventual winner was knocked down by a powerful punch squarely on the face in the initial stage, standing uncertainly to his position on the count of eight.
But the Manchester United-red equipment of Arthur soon came forward again. On the opposite side, the opponent - in competitive blue - was injured soon, significant bruising developing under his eye as the pair traded hard punches.
The challenger, 27, connected with strong blows but Arthur, 34, fought fire with fire with stinging uppercuts as the pace rarely dropped over twelve rounds.
Arthur barely secured the verdict with official scores of even score, 115-113 and 115-112, but both fighters were determined and courageous, everything their inspiration the legend stood for.
