British light-heavyweight Joshua Buatsi made a winning yet underwhelming debut to life under recent management with a decision win over Parker in the Manchester arena.
The Olympic medal winner's punch output likely gained him a split decision in a tight 10-round fight at the boxing arena.
But Parker found success, landing some telling individual punches in a match where the techniques never quite clicked, with constant grabbing marring the action.
Two judges had it 96-94 in favor to the winner, with the final judge a 95-95 draw.
"It wasn't my best performance but what defines champions? They win," the victor commented in the ringside as he was greeted with boos from the crowd.
The Londoner bounced back after his first professional defeat - an thrilling contest with Callum Smith in last winter - to earn a twentieth professional win.
Various analysts thought Parker, 31, merited the victory but the Derbyshire fighter experienced the second setback of his 28-bout career.
The Champion Claims Triumph in Slow Match
His career development has plateaued in recent times through promotional disputes and extended periods of inactivity.
Once tipped as the rising talent of the British fight scene, he now is positioned trying to achieve expectations - possibly in what appears to be a ultimate opportunity - under new promotional backing.
He began cautiously, with neither fighter willing to be aggressive in a cagey early stages.
Parker, whose single earlier setback came at super-middleweight against John Ryder in 2022 after breaking his hand in the contest, was competing in only his second fight at the higher weight class.
By the halfway stage, Parker had kept it close, making life awkward for Buatsi, who operated with a solid defense to barely take the cleaner rounds.
The spectators' acceptance decreased during frequent holding, with the challenger falling to the floor several times during close exchanges.
Parker ended stronger, but many rounds were challenging to assess. Nevertheless, ex-champion Carl Frampton - commentating on Dazn - had it significantly in favor for the challenger, scoring for Buatsi just one round.
Buatsi moves on, but with the likes of power puncher David Benavidez, and Russian star Dmitry Bivol leading the category, the journey to titles - judging by this showing - still looks steep.
Ricky Hatton Memorial Fuels All-Manchester Battle
In the secondary fight, Arthur overcame Rea by divided decision in a excellent local rivalry battle to capture the continental title.
The arena became quiet for a extremely emotional memorial prior to the bout as ten bells tolled in honor of local hero Ricky Hatton, who succumbed at age 46 in September.
It was the premier televised boxing event in the location since Hatton's passing. As the final chime disappeared, the quiet transitioned to a roar of "there's just one Hatton".
The action inside the ropes matched the feeling. The eventual winner was knocked down by a powerful punch directly to the face in the initial stage, rising shakily to his feet on the referee's count.
But the traditional red attire of the fighter soon pressed ahead again. Across the ring, the opponent - in rival City blue - was marked up soon, heavy swelling developing under his face as the pair landed powerful shots.
Rea, twenty-seven, connected with strong blows but the champion, thirty-four, fought fire with fire with powerful upward punches as the action hardly diminished over the full distance.
The victor barely secured the verdict with judges scorecards of 114-114, 115-113 and wider gap, but the two athletes were persistent and brave, all the qualities their idol the legend stood for.
