UK light-heavyweight Joshua Buatsi secured a victory yet underwhelming start to life under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's activity level seemingly secured him a split decision in a tight ten-round bout at the Co-op Live Arena.
But the challenger landed significant shots, connecting with powerful clean blows in a fight where the styles failed to mesh, with excessive holding disrupting the contest.
The officials had it 96-94 for to the winner, with the remaining official a 95-95.
"It wasn't my finest showing but what defines champions? They succeed," the victor commented in the ringside as he was met by negative reactions from the spectators.
The Londoner got back to winning after his first professional defeat - an epic battle with his previous opponent in last winter - to earn a 20th career victory.
Some ringside observers thought the challenger, 31, merited the decision but the Derbyshire fighter experienced the second loss of his 28-fight career.
The Champion Earns Win in Uneventful Encounter
His career development has stalled in recent times through contract problems and considerable time of lack of fights.
Earlier forecast as the next star of British boxing, he now stands trying to fulfil that early promise - possibly in what seems like a last chance - under recent support.
He started carefully, with neither fighter prepared to take risks in a tentative opening few rounds.
Parker, whose only previous defeat came at super middleweight against his former opponent in 2022 after breaking his hand mid-fight, was making just his second fight at light heavyweight.
By the middle rounds, Parker had stayed in contention, creating difficulties for the favorite, who operated with a protective stance to narrowly win the better periods.
The crowd's patience decreased during regular grabbing, with the challenger falling to the floor various instances during close exchanges.
The underdog ended stronger, but many rounds were difficult to score. Although, previous titleholder Carl Frampton - commentating on the broadcast network - had it significantly in favor for Parker, awarding Buatsi just a single round.
The victor advances, but with the caliber of power puncher Benavidez, and Russian star Dmitry Bivol leading the category, the path to championship success - judging by this showing - still appears challenging.
Ricky Hatton Memorial Motivates Manchester Derby Battle
In the secondary fight, Lyndon Arthur overcame Rea by split-decision in a excellent local rivalry fight to win the European light-heavyweight title.
The stadium became quiet for a profoundly moving homage ahead of the contest as 10 bells rang in honor of Manchester's own Ricky Hatton, who succumbed at age 46 in September.
It was the first major televised boxing event in the location since the passing of Hatton. As the last bell disappeared, the stillness gave way to a roar of "there's only one Hatton".
The fighting within the ring reflected the emotion. Arthur was knocked down by a significant blow flush on the face in the initial stage, standing uncertainly to his footing on the referee's count.
But the Manchester United-red shorts of the fighter soon came forward again. On the opposite side, the opponent - in opposing blue - was damaged early, heavy swelling forming under his orbital area as the pair traded hard punches.
Rea, 27, delivered effective strikes but the champion, 34, matched the intensity with powerful upward punches as the pace hardly diminished over the full distance.
Arthur barely secured the verdict with judges scorecards of 114-114, narrow lead and wider gap, but each competitor were determined and brave, all the qualities their inspiration the legend represented.
