British light-heavyweight Joshua Buatsi made a winning yet unimpressive start to life under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic medal winner's work rate appeared to earn him a majority decision in a close 10-round bout at the boxing arena.
But Parker landed significant shots, connecting with powerful clean blows in a fight where the styles never quite clicked, with constant grabbing spoiling the action.
Two judges had it 96-94 to the winner, with the final judge a 95-95 draw.
"It wasn't my finest performance but how do champions respond? They win," Buatsi commented in the ringside as he was greeted with negative reactions from the spectators.
The British fighter bounced back after his maiden career setback - an epic battle with Callum Smith in February - to secure a 20th career victory.
Various analysts felt Parker, 31, deserved the decision but the contender experienced the second setback of his 28-bout career.
Buatsi Claims Victory in Dull Affair
His career development has plateaued in the past few years through management issues and extended periods of lack of fights.
Earlier forecast as the next star of British boxing, he now finds himself trying to realize his potential - perhaps in what seems like a last chance - under fresh management.
He began cautiously, with the two boxers ready to be aggressive in a calculated opening few rounds.
Parker, whose single earlier setback came at the lower weight class against his former opponent in 2022 after breaking his hand mid-fight, was competing in only his second appearance at the higher weight class.
By the fight's midpoint, Parker had stayed in contention, creating difficulties for the favorite, who operated with a protective stance to barely take the more effective rounds.
The spectators' acceptance diminished amid regular grabbing, with Parker going down several times during close exchanges.
The underdog closed well, but several stanzas were challenging to assess. However, ex-champion Carl Frampton - working with Dazn - had it particularly wide for the challenger, giving Buatsi just a single round.
The victor progresses, but with the likes of hard hitter Benavidez, and Russian star Dmitry Bivol dominating the weight class, the road to world glory - judging by this showing - still appears challenging.
Ricky Hatton Memorial Inspires Local Rivalry Fight
In the co-main event, Arthur overcame Rea by divided decision in a terrific all-Manchester fight to win the European light heavyweight championship.
The venue grew hushed for a profoundly moving homage before the fight as a bell ceremony sounded in remembrance of local hero Ricky Hatton, who died at age 46 in recently.
It was the first major televised fight night in the location since Hatton's passing. As the final chime faded, the silence became a shout of "there's just one Ricky Hatton".
The fighting during the bout equaled the sentiment. Arthur was sent to the canvas by a left hook flush on the jaw in the early round, getting up unsteadily to his position on the eight count.
But the red-colored equipment of the fighter soon pressed ahead again. Across the ring, Rea - in rival City blue - was marked up quickly, significant bruising forming under his eye as the duo exchanged powerful shots.
The challenger, twenty-seven, connected with strong blows but Arthur, 34, matched the intensity with stinging uppercuts as the intensity seldom decreased over 12 rounds.
Arthur edged the decision with judges scorecards of 114-114, close margin and 115-112, but the two athletes were persistent and full of heart, everything their hero Hatton embodied.
