British light heavyweight Joshua Buatsi earned a win yet underwhelming debut to his career under recent management with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's activity level likely gained him a majority decision in a close ten-round fight at the Co-op Live Arena.
But Parker landed significant shots, connecting with powerful single shots in a contest where the techniques failed to mesh, with frequent clinching marring the contest.
The officials judged it 96-94 in favor to Buatsi, with the remaining official a even score.
"It wasn't my greatest showing but what do champions do? They succeed," the victor said in the arena as he was faced boos from the crowd.
The Londoner got back to winning after his maiden career setback - an thrilling contest with Callum Smith in February - to secure a 20th career victory.
Various analysts thought Parker, 31, deserved the win but the British boxer endured the second defeat of his 28-bout career.
The Champion Claims Triumph in Slow Affair
Buatsi's progress has slowed in recent times through contract problems and considerable time of inactivity.
Earlier forecast as the rising talent of UK boxing, he now stands trying to fulfil that early promise - maybe in what seems like a ultimate opportunity - under new promotional backing.
He opened tentatively, with both competitors prepared to take risks in a tentative initial rounds.
Parker, whose sole prior loss came at the lower weight class against his former opponent in 2022 after damaging his hand during the bout, was making just his second fight at light-heavyweight.
By the fight's midpoint, the underdog had stayed in contention, creating difficulties for Buatsi, who utilized a protective stance to edge the cleaner stanzas.
The crowd's patience diminished because of frequent holding, with the challenger going down multiple occasions during close exchanges.
Parker ended stronger, but many rounds were difficult to score. However, ex-champion Carl Frampton - working with the broadcast network - had it particularly wide for Parker, awarding Buatsi just only one stanza.
The victor progresses, but with the presence of power puncher Benavidez, and Russian champion Bivol dominating the weight class, the journey to titles - on this evidence - still seems difficult.
Ricky Hatton Homage Motivates Manchester Derby Contest
In the secondary fight, Arthur beat Rea by split-decision in a outstanding all-Manchester fight to capture the European light-heavyweight championship.
The arena became quiet for a extremely emotional homage before the fight as a bell ceremony sounded in honor of local hero Ricky Hatton, who died 46 years old in last month.
It was the premier televised match card in the urban center since Hatton's passing. As the concluding sound ended, the silence gave way to a roar of "there's only one Hatton".
The competition during the bout equaled the emotion. Arthur was knocked down by a left hook flush on the chin in the early round, getting up unsteadily to his footing on the referee's count.
But the red-colored shorts of Arthur soon pressed ahead again. From the other corner, Rea - in rival City blue - was injured soon, significant bruising forming under his face as the fighters exchanged hard punches.
Rea, 27, landed power punches but the champion, 34, fought fire with fire with powerful upward punches as the action seldom decreased over twelve rounds.
The victor edged the result with official scores of 114-114, close margin and 115-112, but both fighters were relentless and brave, exactly what their idol Hatton stood for.
