UK light heavyweight Joshua Buatsi secured a victory yet underwhelming beginning to his career under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's punch output appeared to earn him a judges' verdict in a competitive 10-round bout at the Manchester venue.
But Parker found success, landing some telling clean blows in a contest where the approaches never quite clicked, with frequent clinching marring the fight.
The officials judged it 96-94 to Buatsi, with the remaining official a even score.
"It wasn't my best performance but what defines champions? They win," the victor stated in the ring as he was met by boos from the spectators.
The British fighter returned to winning ways after his first professional defeat - an thrilling contest with Callum Smith in February - to earn a 20th career victory.
Some ringside observers thought the challenger, 31, deserved the win but the British boxer experienced the second defeat of his 28-match career.
The Champion Claims Victory in Dull Affair
Buatsi's progress has plateaued in recent years through promotional disputes and considerable time of inactivity.
Earlier forecast as the rising talent of British boxing, he now stands trying to fulfil that early promise - maybe in what seems like a ultimate opportunity - under recent support.
He started carefully, with neither fighter willing to take risks in a calculated opening few rounds.
Parker, whose sole prior loss came at super middleweight against his former opponent in 2022 after damaging his hand during the bout, was participating in his second fight at light-heavyweight.
By the fight's midpoint, the underdog had made it competitive, causing problems for the favorite, who utilized a protective stance to barely take the more effective stanzas.
The crowd's patience diminished amid constant clinching, with Parker going down several times during clinches.
The underdog finished the stronger, but multiple periods were difficult to score. However, previous titleholder Carl Frampton - commentating on the broadcast network - had it strongly leaning for Parker, scoring for Buatsi just one round.
The victor moves on, but with the likes of heavy hitter David Benavidez, and Russian champion Dmitry Bivol leading the category, the path to championship success - judging by this showing - still appears challenging.
Ricky Hatton Memorial Inspires All-Manchester Battle
In the secondary fight, Lyndon Arthur defeated Bradley Rea by close verdict in a terrific all-Manchester contest to capture the European light heavyweight belt.
The arena became quiet for a extremely emotional homage ahead of the contest as a bell ceremony rang in remembrance of the city's native Hatton, who passed away at age 46 in last month.
It was the initial significant broadcast match card in the urban center since Hatton's passing. As the last bell ended, the quiet transitioned to a shout of "there's just one Hatton".
The action during the bout matched the emotion. The eventual winner was sent to the canvas by a significant blow flush on the jaw in the initial stage, getting up unsteadily to his position on the referee's count.
But the red-colored equipment of the fighter soon came forward again. On the opposite side, Rea - in competitive blue - was damaged soon, heavy swelling appearing under his orbital area as the duo exchanged hard punches.
The challenger, twenty-seven, delivered effective strikes but the champion, thirty-four, fought fire with fire with powerful upward punches as the action rarely dropped over 12 rounds.
Arthur narrowly won the verdict with official scores of draw, 115-113 and 115-112, but both fighters were persistent and full of heart, all the qualities their hero Hatton embodied.
