British light heavyweight Joshua Buatsi secured a victory yet disappointing 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 close ten-round contest at the boxing arena.
But Parker had his moments, delivering effective clean blows in a match where the approaches didn't complement each other, with excessive holding spoiling the action.
Two judges had it 96-94 in favor to Buatsi, with the final judge a 95-95 draw.
"It wasn't my greatest performance but what defines champions? They win," the victor said in the arena as he was greeted with disapproval from the crowd.
The Londoner bounced back after his initial pro loss - an epic battle with Callum Smith in February - to earn a 20th pro success.
Various analysts thought Parker, 31, merited the victory but the British boxer experienced the second setback of his 28-fight career.
Buatsi Claims Win in Uneventful Affair
His career development has slowed in the past few years through management issues and considerable time of inactivity.
Once tipped as the future of UK boxing, he now stands trying to realize his potential - perhaps in what seems like a final roll of the dice - under new promotional backing.
He opened tentatively, with the two boxers prepared to overcommit in a cagey opening few rounds.
The challenger, whose single earlier setback came at the lower weight class against John Ryder in 2022 after breaking his hand mid-fight, was competing in only his second fight at light heavyweight.
By the fight's midpoint, Parker had made it competitive, making life awkward for the favorite, who utilized a solid defense to edge the cleaner periods.
The spectators' acceptance decreased because of constant clinching, with the challenger slipping to the canvas multiple occasions during close exchanges.
Parker ended stronger, but many rounds were hard to judge. However, ex-champion the boxing analyst - commentating on the broadcast network - had it particularly wide for Parker, giving Buatsi just one round.
The victor progresses, but with the likes of power puncher Benavidez, and Russian star Bivol dominating the weight class, the journey to titles - based on this performance - still looks steep.
Hatton Memorial Fuels Local Rivalry Battle
In the secondary fight, Arthur beat Bradley Rea by close verdict in a terrific all-Manchester contest to capture the European light heavyweight belt.
The venue grew hushed for a deeply poignant homage prior to the bout as 10 bells sounded in honor of Manchester's own Hatton, who succumbed 46 years old in recently.
It was the first major televised boxing event in the city since the passing of Hatton. As the final chime ended, the silence transitioned to a shout of "only one Ricky Hatton".
The competition during the bout equaled the sentiment. The eventual winner was floored by a significant blow flush on the jaw in the early round, getting up unsteadily to his feet on the eight count.
But the Manchester United-red shorts of Arthur soon advanced again. From the other corner, the opponent - in rival City blue - was damaged quickly, significant bruising developing under his face as the fighters landed powerful shots.
Rea, 27, landed power punches but the champion, 34, fought fire with fire with effective uppercuts as the intensity rarely dropped over the full distance.
Arthur edged the result with judges scorecards of even score, close margin and clearer margin, but each competitor were relentless and full of heart, exactly what their hero Hatton embodied.
