British light heavyweight Joshua Buatsi earned a win yet unimpressive beginning to his career under recent management with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's work rate likely gained him a judges' verdict in a close ten-round bout at the Co-op Live Arena.
But the challenger had his moments, delivering effective single shots in a contest where the styles didn't complement each other, with frequent clinching spoiling the fight.
The officials scored it 96-94 to Buatsi, with the remaining official a 95-95.
"It wasn't my finest showing but how do champions respond? They win," Buatsi stated in the arena as he was met by negative reactions from the spectators.
The British fighter returned to winning ways after his maiden career setback - an memorable fight with his previous opponent in February - to claim a 20th pro success.
Some ringside observers felt the challenger, 31, deserved the victory but the contender experienced the second defeat of his 28-match career.
The Champion Secures Win in Uneventful Match
The fighter's advancement has slowed in recent times through promotional disputes and long spells of lack of fights.
Once tipped as the rising talent of British boxing, he now stands trying to fulfil that early promise - perhaps in what seems like a ultimate opportunity - under fresh management.
He began cautiously, with both competitors ready to take risks in a tentative opening few rounds.
Parker, whose only previous defeat came at super-middleweight against John Ryder in 2022 after damaging his hand during the bout, was competing in only his second fight at light heavyweight.
By the fight's midpoint, Parker had stayed in contention, causing problems for the favorite, who worked behind a protective stance to barely take the better stanzas.
The audience's tolerance diminished during frequent holding, with the challenger falling to the floor various instances during close exchanges.
Parker closed well, but several stanzas were challenging to assess. Although, ex-champion the boxing analyst - working with Dazn - had it strongly leaning for the challenger, scoring for Buatsi just only one stanza.
Buatsi moves on, but with the likes of heavy hitter David Benavidez, and Russian star Dmitry Bivol leading the category, the road to world glory - based on this performance - still looks steep.
Ricky Hatton Memorial Motivates Local Rivalry Fight
In the co-main event, Lyndon Arthur beat Rea by split-decision in a excellent Manchester vs Manchester contest to win the European light-heavyweight championship.
The stadium became quiet for a deeply poignant homage before the fight as ten bells tolled in remembrance of Manchester's own Hatton, who succumbed aged 46 in last month.
It was the first major televised match card in the location since Hatton's passing. As the last bell disappeared, the stillness became a shout of "only one Hatton".
The fighting during the bout equaled the feeling. Arthur was sent to the canvas by a left hook squarely on the jaw in the initial stage, rising shakily to his position on the referee's count.
But the Manchester United-red shorts of Arthur soon pressed ahead again. From the other corner, the opponent - in competitive blue - was marked up early, significant bruising developing under his orbital area as the duo traded hard punches.
The challenger, 27, landed power punches but the champion, thirty-four, fought fire with fire with effective uppercuts as the pace rarely dropped over 12 rounds.
Arthur narrowly won the result with scoring tallies of even score, close margin and clearer margin, but both fighters were determined and brave, all the qualities their inspiration the legend stood for.
