UK light heavyweight Joshua Buatsi secured a victory yet unimpressive debut to life under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's punch output appeared to earn him a majority decision in a competitive ten-round contest at the Manchester venue.
But Parker found success, landing some telling clean blows in a contest where the styles didn't complement each other, with frequent clinching spoiling the action.
The scoring judges judged it 96-94 in favor to Buatsi, with the third a 95-95 draw.
"It wasn't my finest performance but what defines champions? They prevail," the victor said in the arena as he was greeted with negative reactions from the spectators.
The British fighter got back to winning after his maiden career setback - an thrilling contest with Callum Smith in last winter - to secure a 20th career victory.
Various analysts felt Parker, 31, merited the win but the British boxer experienced the second loss of his 28-fight career.
The Champion Earns Win in Dull Affair
The fighter's advancement has slowed in recent times through contract problems and long spells of inactivity.
Once tipped as the future of UK boxing, he now finds himself trying to fulfil that early promise - perhaps in what seems like a last chance - under fresh management.
He started carefully, with neither fighter ready to be aggressive in a calculated early stages.
Parker, whose only previous defeat came at super middleweight against John Ryder in 2022 after damaging his hand in the contest, was making just his second bout at light-heavyweight.
By the middle rounds, Parker had stayed in contention, making life awkward for Buatsi, who worked behind a protective stance to barely take the better stanzas.
The crowd's patience decreased amid regular grabbing, with the challenger going down multiple occasions during grappling.
The underdog finished the stronger, but multiple periods were challenging to assess. Although, ex-champion the boxing analyst - working with Dazn - had it strongly leaning for Parker, giving Buatsi just a single round.
The victor progresses, but with the presence of hard hitter Benavidez, and Russian champion Bivol dominating the weight class, the road to world glory - on this evidence - still looks steep.
Hatton Memorial Motivates Local Rivalry Fight
In the secondary fight, Lyndon Arthur overcame Rea by divided decision in a terrific Manchester vs Manchester contest to win the European light heavyweight championship.
The arena became quiet for a deeply poignant tribute before the fight as ten bells rang in honor of the city's native Ricky Hatton, who succumbed at age 46 in September.
It was the premier televised match card in the city since the passing of Hatton. As the concluding sound faded, the quiet transitioned to a shout of "only one Hatton".
The fighting inside the ropes reflected the emotion. Arthur was sent to the canvas by a powerful punch squarely on the chin in the second round, getting up unsteadily to his position on the eight count.
But the red-colored equipment of Arthur soon pressed ahead again. Across the ring, the opponent - in competitive blue - was marked up early, significant bruising forming under his orbital area as the pair landed punishing blows.
The challenger, twenty-seven, landed power punches but Arthur, thirty-four, fought fire with fire with powerful upward punches as the action hardly diminished over 12 rounds.
The victor narrowly won the result with judges scorecards of even score, 115-113 and wider gap, but both fighters were determined and full of heart, exactly what their idol the legend stood for.
