UK light heavyweight Joshua Buatsi earned a win yet unimpressive debut to life under recent management with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's work rate appeared to earn him a judges' verdict in a tight 10-round bout at the Co-op Live Arena.
But Parker landed significant shots, delivering effective individual punches in a contest where the styles didn't complement each other, with excessive holding spoiling the fight.
The officials scored it 96-94 to the winner, with the remaining official a 95-95.
"It wasn't my greatest showing but how do champions respond? They succeed," Buatsi commented in the arena as he was greeted with negative reactions from the spectators.
The British fighter got back to winning after his first professional defeat - an thrilling contest with his previous opponent in last winter - to earn a twentieth professional win.
Some ringside observers believed the challenger, thirty-one, deserved the win but the Derbyshire fighter endured the second loss of his 28-match career.
The Champion Earns Win in Uneventful Match
The fighter's advancement has plateaued in recent years through management issues and extended periods of lack of fights.
Previously predicted as the rising talent of British boxing, he now is positioned trying to achieve expectations - possibly in what appears to be a final roll of the dice - under fresh management.
He started carefully, with the two boxers ready to overcommit in a calculated initial rounds.
The challenger, whose single earlier setback came at super middleweight against John Ryder in 2022 after damaging his hand in the contest, was making just his second bout at light-heavyweight.
By the middle rounds, Parker had made it competitive, causing problems for Buatsi, who worked behind a protective stance to narrowly win the better rounds.
The crowd's patience diminished during constant clinching, with the challenger falling to the floor various instances during grappling.
Parker finished the stronger, but several stanzas were challenging to assess. However, previous titleholder the boxing analyst - working with the broadcast network - had it particularly wide for Parker, scoring for Buatsi just a single round.
The victor moves on, but with the likes of heavy hitter David Benavidez, and Russian champion Dmitry Bivol ruling the division, the journey to titles - on this evidence - still appears challenging.
Ricky Hatton Memorial Motivates Manchester Derby Battle
In the secondary fight, Lyndon Arthur defeated Bradley Rea by divided decision in a terrific Manchester vs Manchester battle to win the continental belt.
The stadium grew hushed for a profoundly moving tribute before the fight as ten bells rang in memory of local hero Hatton, who died aged 46 in recently.
It was the initial significant broadcast boxing event in the city since Hatton's death. As the last bell faded, the quiet became a chant of "only one Hatton".
The action during the bout equaled the emotion. The eventual winner was sent to the canvas by a left hook flush on the jaw in the initial stage, standing uncertainly to his footing on the referee's count.
But the Manchester United-red equipment of the fighter soon advanced again. From the other corner, the opponent - in opposing blue - was injured soon, significant bruising developing under his face as the duo traded powerful shots.
Rea, twenty-seven, connected with strong blows but Arthur, 34, responded in kind with powerful upward punches as the intensity hardly diminished over the full distance.
Arthur narrowly won the decision with judges scorecards of 114-114, close margin and clearer margin, but the two athletes were determined and full of heart, everything their inspiration the legend represented.
