UK light-heavyweight Joshua Buatsi made a winning yet unimpressive debut to life under new promoters with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's work rate appeared to earn him a majority decision in a competitive ten-round contest at the Co-op Live Arena.
But the challenger found success, landing some telling single shots in a match where the techniques failed to mesh, with excessive holding spoiling the contest.
The officials had it 96-94 in favor to the winner, with the third a 95-95 draw.
"It wasn't my greatest performance but what do champions do? They prevail," Buatsi said in the arena as he was greeted with disapproval from the audience.
The British fighter got back to winning after his first professional defeat - an memorable fight with Callum Smith in February - to earn a 20th pro success.
Several boxing experts thought Parker, 31, merited the victory but the contender experienced the second defeat of his 28-match career.
The Champion Secures Victory in Slow Affair
The fighter's advancement has plateaued in the past few years through contract problems and extended periods of lack of fights.
Earlier forecast as the rising talent of the British fight scene, he now stands trying to fulfil that early promise - possibly in what feels like a ultimate opportunity - under new promotional backing.
He started carefully, with both competitors willing to take risks in a cagey opening few rounds.
The challenger, whose sole prior loss came at super-middleweight against his former opponent in 2022 after suffering a hand injury mid-fight, was participating in his second fight at light heavyweight.
By the halfway stage, the underdog had stayed in contention, making life awkward for Buatsi, who operated with a protective stance to barely take the better periods.
The crowd's patience diminished amid regular grabbing, with the challenger going down multiple occasions during clinches.
The underdog finished the stronger, but several stanzas were hard to judge. Nevertheless, previous titleholder the boxing analyst - providing analysis for the broadcast network - had it significantly in favor for the challenger, giving Buatsi just one round.
Buatsi moves on, but with the presence of power puncher Benavidez, and Russian champion Bivol leading the category, the journey to titles - on this evidence - still looks steep.
Ricky Hatton Memorial Fuels All-Manchester Contest
In the co-main event, Lyndon Arthur overcame Rea by close verdict in a terrific local rivalry fight to win the European light-heavyweight title.
The arena grew hushed for a deeply poignant homage ahead of the contest as 10 bells rang in remembrance of Manchester's own Hatton, who succumbed aged 46 in last month.
It was the first major televised boxing event in the location since Hatton's death. As the final chime disappeared, the stillness transitioned to a shout of "there's only one Ricky Hatton".
The fighting within the ring reflected the emotion. Arthur was knocked down by a powerful punch squarely on the chin in the initial stage, standing uncertainly to his feet on the count of eight.
But the traditional red equipment of the fighter soon pressed ahead again. From the other corner, Rea - in opposing blue - was damaged soon, significant bruising developing under his face as the duo landed punishing blows.
Rea, twenty-seven, connected with strong blows but the champion, thirty-four, fought fire with fire with stinging uppercuts as the pace hardly diminished over the full distance.
The victor edged the verdict with official scores of 114-114, close margin and 115-112, but the two athletes were determined and full of heart, all the qualities their idol the legend stood for.
