UK light-heavyweight Joshua Buatsi secured a victory yet disappointing debut to his career under new promoters with a decision win over Parker in Manchester.
The Olympic bronze medallist's work rate likely gained him a majority decision in a competitive ten-round contest at the Co-op Live Arena.
But the challenger found success, connecting with powerful single shots in a match where the techniques never quite clicked, with frequent clinching marring the fight.
Two judges had it 96-94 to the winner, with the remaining official a 95-95 draw.
"It wasn't my best display but what defines champions? They win," Buatsi said in the arena as he was faced disapproval from the audience.
The British fighter got back to winning after his maiden career setback - an memorable fight with Callum Smith in last winter - to earn a twentieth professional win.
Various analysts felt the challenger, thirty-one, deserved the victory but the British boxer experienced the second defeat of his 28-match professional journey.
The Champion Secures Victory in Slow Affair
The fighter's advancement has slowed in recent years through promotional disputes and long spells of inactivity.
Once tipped as the rising talent of the British fight scene, he now stands trying to fulfil that early promise - perhaps in what feels like a ultimate opportunity - under recent support.
He began cautiously, with neither fighter prepared to take risks in a calculated initial rounds.
Parker, whose sole prior loss came at the lower weight class against his former opponent in 2022 after breaking his hand during the bout, was making just his second bout at light heavyweight.
By the middle rounds, the underdog had made it competitive, creating difficulties for the favorite, who utilized a solid defense to edge the better stanzas.
The crowd's patience diminished amid frequent holding, with the challenger going down various instances during grappling.
The underdog finished the stronger, but several stanzas were difficult to score. However, former world champion Carl Frampton - commentating on the broadcast network - had it strongly leaning for the challenger, scoring for Buatsi just only one stanza.
Buatsi advances, but with the presence of heavy hitter David Benavidez, and Russian star Dmitry Bivol dominating the weight class, the path to championship success - based on this performance - still looks steep.
Ricky Hatton Tribute Inspires Manchester Derby Contest
In the supporting bout, Arthur beat Rea by split-decision in a outstanding local rivalry contest to capture the continental belt.
The venue fell silent for a extremely emotional memorial prior to the bout as ten bells tolled in honor of the city's native Hatton, who passed away aged 46 in recently.
It was the premier televised fight night in the location since the passing of Hatton. As the concluding sound disappeared, the stillness gave way to a shout of "only one Ricky Hatton".
The action during the bout equaled the emotion. The eventual winner was floored by a powerful punch squarely on the jaw in the initial stage, standing uncertainly to his position on the count of eight.
But the Manchester United-red equipment of the fighter soon pressed ahead again. From the other corner, the opponent - in rival City blue - was injured soon, heavy swelling developing under his orbital area as the fighters traded punishing blows.
The challenger, 27, connected with strong blows but the champion, 34, responded in kind with stinging uppercuts as the action seldom decreased over twelve rounds.
The victor barely secured the result with official scores of draw, close margin and 115-112, but the two athletes were relentless and courageous, everything their idol Hatton embodied.
