UK light heavyweight Joshua Buatsi made a winning yet unimpressive start to life under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic medal winner's punch output appeared to earn him a majority decision in a tight 10-round bout at the Co-op Live Arena.
But Parker had his moments, delivering effective clean blows in a match where the styles failed to mesh, with constant grabbing disrupting the action.
The scoring judges judged it 96-94 in favor to Buatsi, with the third a 95-95.
"It wasn't my best showing but how do champions respond? They win," the victor commented in the arena as he was faced disapproval from the audience.
The British fighter returned to winning ways after his first professional defeat - an thrilling contest with his previous opponent in last winter - to claim a 20th pro success.
Various analysts felt the challenger, thirty-one, should have been awarded the win but the contender suffered the second loss of his 28-match career.
Buatsi Claims Triumph in Dull Affair
The fighter's advancement has stalled in the past few years through promotional disputes and extended periods of inactivity.
Once tipped as the rising talent of the British fight scene, he now finds himself trying to fulfil that early promise - possibly in what feels like a final roll of the dice - under fresh management.
He started carefully, with both competitors ready to be aggressive in a cagey opening few rounds.
Parker, whose sole prior loss came at super middleweight against his former opponent in 2022 after breaking his hand during the bout, was participating in his second appearance at light-heavyweight.
By the halfway stage, Parker had kept it close, making life awkward for the favorite, who operated with a solid defense to narrowly win the cleaner rounds.
The crowd's patience wore thin because of constant clinching, with Parker slipping to the canvas various instances during close exchanges.
Parker finished the stronger, but several stanzas were difficult to score. However, previous titleholder the boxing analyst - commentating on the broadcast network - had it particularly wide for the challenger, awarding Buatsi just one round.
The victor progresses, but with the likes of power puncher David Benavidez, and Russian champion Dmitry Bivol leading the category, the road to world glory - on this evidence - still appears challenging.
Hatton Tribute Motivates Manchester Derby Fight
In the secondary fight, Arthur beat Rea by divided decision in a terrific all-Manchester battle to secure the continental championship.
The stadium fell silent for a profoundly moving homage before the fight as ten bells tolled in honor of the city's native Ricky Hatton, who succumbed aged 46 in September.
It was the premier televised boxing event in the city since Hatton's passing. As the concluding sound disappeared, the stillness gave way to a chant of "there's just one Ricky Hatton".
The competition inside the ropes equaled the feeling. Arthur was knocked down by a powerful punch squarely on the chin in the early round, rising shakily to his position on the count of eight.
But the traditional red equipment of Arthur soon came forward again. From the other corner, the opponent - in opposing blue - was damaged quickly, significant bruising developing under his eye as the pair traded punishing blows.
The challenger, twenty-seven, landed power punches but Arthur, 34, responded in kind with stinging uppercuts as the action rarely dropped over the full distance.
The victor narrowly won the verdict with scoring tallies of draw, 115-113 and clearer margin, but each competitor were relentless and full of heart, everything their inspiration Hatton stood for.
