British light heavyweight Joshua Buatsi earned a win yet underwhelming debut to life under recent management with a decision win over Parker in Manchester.
The Olympic bronze medallist's punch output appeared to earn him a judges' verdict in a close 10-round bout at the Manchester venue.
But Parker found success, connecting with powerful clean blows in a contest where the approaches didn't complement each other, with excessive holding spoiling the fight.
The officials scored it 96-94 to the winner, with the third a even score.
"It wasn't my best showing but how do champions respond? They succeed," Buatsi commented in the ringside as he was greeted with boos from the spectators.
The British fighter returned to winning ways after his first professional defeat - an thrilling contest with Callum Smith in last winter - to claim a 20th career victory.
Various analysts believed the challenger, thirty-one, deserved the win but the Derbyshire fighter experienced the second defeat of his 28-match career.
Buatsi Claims Win in Dull Affair
Buatsi's progress has plateaued in recent times through contract problems and long spells of inactivity.
Once tipped as the future of the British fight scene, he now is positioned trying to fulfil that early promise - maybe in what appears to be a last chance - under new promotional backing.
He started carefully, with neither fighter ready to overcommit in a cagey initial rounds.
The challenger, whose only previous defeat came at super middleweight against his former opponent in 2022 after suffering a hand injury mid-fight, was competing in only his second bout at light heavyweight.
By the fight's midpoint, Parker had stayed in contention, causing problems for Buatsi, who utilized a solid defense to barely take the more effective periods.
The crowd's patience decreased because of frequent holding, with the challenger slipping to the canvas multiple occasions during clinches.
Parker closed well, but multiple periods were hard to judge. Although, ex-champion the boxing analyst - working with the broadcast network - had it strongly leaning for Parker, awarding Buatsi just only one stanza.
The victor progresses, but with the presence of power puncher Benavidez, and Russian star Dmitry Bivol ruling the division, the path to championship success - based on this performance - still seems difficult.
Hatton Memorial Fuels Local Rivalry Contest
In the co-main event, Arthur defeated Rea by divided decision in a terrific Manchester vs Manchester contest to secure the European light-heavyweight title.
The stadium became quiet for a profoundly moving tribute before the fight as a bell ceremony rang in remembrance of Manchester's own Ricky Hatton, who passed away 46 years old in last month.
It was the premier televised match card in the city since Hatton's death. As the concluding sound disappeared, the silence transitioned to a shout of "there's just one Ricky Hatton".
The action within the ring equaled the sentiment. Arthur was knocked down by a left hook directly to the jaw in the second round, standing uncertainly to his position on the count of eight.
But the red-colored shorts of the fighter soon pressed ahead again. On the opposite side, Rea - in opposing blue - was injured soon, heavy swelling developing under his orbital area as the fighters landed powerful shots.
Rea, twenty-seven, connected with strong blows but the champion, 34, fought fire with fire with powerful upward punches as the pace hardly diminished over the full distance.
The victor barely secured the decision with judges scorecards of even score, 115-113 and clearer margin, but each competitor were determined and brave, exactly what their hero Hatton embodied.
