British light-heavyweight Joshua Buatsi made a winning yet underwhelming debut to his career under recent management with a decision win over Parker in Manchester.
The Olympic bronze medallist's work rate seemingly secured him a judges' verdict in a tight ten-round fight at the Co-op Live Arena.
But the challenger found success, delivering effective single shots in a fight where the styles didn't complement each other, with excessive holding marring the fight.
Two judges judged it 96-94 to the winner, with the remaining official a even score.
"It wasn't my greatest performance but what defines champions? They win," the victor stated in the arena as he was faced negative reactions from the crowd.
The British fighter bounced back after his initial pro loss - an epic battle with his previous opponent in February - to claim a 20th pro success.
Various analysts felt Parker, 31, should have been awarded the victory but the British boxer suffered the second defeat of his 28-bout professional journey.
The Champion Secures Victory in Slow Match
The fighter's advancement has plateaued in recent times through management issues and extended periods of lack of fights.
Previously predicted as the future of the British fight scene, he now stands trying to achieve expectations - maybe in what appears to be a ultimate opportunity - under recent support.
He began cautiously, with neither fighter prepared to be aggressive in a tentative early stages.
Parker, whose only previous defeat came at super-middleweight against his former opponent in 2022 after breaking his hand in the contest, was competing in only his second fight at light heavyweight.
By the middle rounds, Parker had stayed in contention, creating difficulties for Buatsi, who utilized a protective stance to edge the more effective periods.
The crowd's patience diminished amid regular grabbing, with the challenger falling to the floor several times during close exchanges.
Parker finished the stronger, but several stanzas were difficult to score. However, ex-champion Carl Frampton - providing analysis for the broadcast network - had it significantly in favor for Parker, scoring for Buatsi just only one stanza.
The victor advances, but with the presence of power puncher David Benavidez, and Russian champion Bivol ruling the division, the journey to titles - based on this performance - still looks steep.
Ricky Hatton Tribute Inspires Local Rivalry Fight
In the co-main event, Lyndon Arthur overcame Rea by close verdict in a outstanding local rivalry contest to secure the continental championship.
The arena grew hushed for a profoundly moving memorial prior to the bout as ten bells rang in honor of local hero Hatton, who died at age 46 in September.
It was the first major televised fight night in the city since the passing of Hatton. As the concluding sound ended, the stillness gave way to a shout of "there's just one Ricky Hatton".
The action within the ring matched the sentiment. The eventual winner was sent to the canvas by a significant blow squarely on the jaw in the initial stage, getting up unsteadily to his position on the eight count.
But the traditional red shorts of Arthur soon came forward again. On the opposite side, Rea - in competitive blue - was marked up early, heavy swelling appearing under his eye as the pair traded punishing blows.
Rea, 27, connected with strong blows but the champion, thirty-four, matched the intensity with powerful upward punches as the intensity hardly diminished over 12 rounds.
The victor narrowly won the decision with official scores of 114-114, 115-113 and 115-112, but both fighters were determined and brave, all the qualities their idol the legend stood for.
