UK light-heavyweight Joshua Buatsi earned a win yet disappointing start to life under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's work rate likely gained him a split decision in a close ten-round fight at the Manchester venue.
But Parker landed significant shots, landing some telling clean blows in a contest where the techniques didn't complement each other, with excessive holding spoiling the fight.
Two judges had it 96-94 for to the winner, with the third a even score.
"It wasn't my best showing but how do champions respond? They succeed," the victor commented in the ring as he was faced disapproval from the audience.
The British fighter bounced back after his initial pro loss - an thrilling contest with his previous opponent in last winter - to claim a 20th pro success.
Some ringside observers thought the challenger, thirty-one, should have been awarded the decision but the contender experienced the second setback of his 28-fight career.
The Champion Earns Win in Slow Affair
The fighter's advancement has plateaued in the past few years through promotional disputes and extended periods of inactivity.
Once tipped as the next star of the British fight scene, he now finds himself trying to fulfil that early promise - perhaps in what appears to be a final roll of the dice - under fresh management.
He opened tentatively, with both competitors ready to overcommit in a calculated opening few rounds.
Parker, whose only previous defeat came at the lower weight class against John Ryder in 2022 after breaking his hand in the contest, was making just his second bout at light heavyweight.
By the halfway stage, Parker had made it competitive, creating difficulties for Buatsi, who utilized a protective stance to barely take the better rounds.
The crowd's patience diminished during frequent holding, with Parker falling to the floor various instances during close exchanges.
The underdog finished the stronger, but multiple periods were hard to judge. Although, ex-champion the boxing analyst - providing analysis for the broadcast network - had it strongly leaning for Parker, giving Buatsi just one round.
Buatsi moves on, but with the caliber of power puncher David Benavidez, and Russian champion Dmitry Bivol leading the category, the road to world glory - on this evidence - still looks steep.
Hatton Tribute Fuels Local Rivalry Fight
In the supporting bout, Arthur overcame Rea by divided decision in a outstanding Manchester vs Manchester contest to win the continental championship.
The arena became quiet for a profoundly moving tribute before the fight as a bell ceremony tolled in honor of Manchester's own Hatton, who died aged 46 in September.
It was the first major televised match card in the urban center since Hatton's passing. As the final chime ended, the quiet transitioned to a chant of "there's just one Ricky Hatton".
The action within the ring matched the feeling. The eventual winner was floored by a powerful punch flush on the jaw in the second round, rising shakily to his footing on the eight count.
But the red-colored attire of Arthur soon came forward again. On the opposite side, Rea - in rival City blue - was injured early, noticeable damage forming under his face as the pair landed punishing blows.
The challenger, twenty-seven, delivered effective strikes but the champion, thirty-four, matched the intensity with stinging uppercuts as the pace seldom decreased over the full distance.
Arthur narrowly won the decision with official scores of draw, narrow lead and clearer margin, but the two athletes were relentless and brave, all the qualities their idol Hatton stood for.
