British light heavyweight Joshua Buatsi earned a win yet unimpressive beginning to his career under new promoters with a points victory over Parker in Manchester.
The Olympic medal winner's punch output likely gained him a majority decision in a close 10-round bout at the Co-op Live Arena.
But Parker found success, connecting with powerful single shots in a contest where the approaches never quite clicked, with excessive holding spoiling the action.
Two judges had it 96-94 in favor to Buatsi, with the remaining official a 95-95 draw.
"It wasn't my best display but how do champions respond? They win," Buatsi stated in the ringside as he was greeted with disapproval from the spectators.
The Londoner returned to winning ways after his initial pro loss - an memorable fight with his previous opponent in February - to claim a twentieth professional win.
Some ringside observers thought Parker, 31, should have been awarded the win but the British boxer experienced the second setback of his 28-match career.
The Champion Claims Triumph in Slow Encounter
Buatsi's progress has slowed in recent times through promotional disputes and extended periods of inactivity.
Once tipped as the rising talent of British boxing, he now finds himself trying to realize his potential - possibly in what feels like a ultimate opportunity - under fresh management.
He opened tentatively, with both competitors willing to take risks in a tentative opening few rounds.
The challenger, whose single earlier setback came at super middleweight against John Ryder in 2022 after suffering a hand injury in the contest, was participating in his second bout at the higher weight class.
By the halfway stage, Parker had kept it close, creating difficulties for the favorite, who operated with a tight guard to barely take the better stanzas.
The audience's tolerance wore thin during frequent holding, with Parker going down several times during close exchanges.
Parker closed well, but multiple periods were challenging to assess. Nevertheless, ex-champion the boxing analyst - providing analysis for Dazn - had it strongly leaning for Parker, giving Buatsi just only one stanza.
Buatsi progresses, but with the likes of heavy hitter David Benavidez, and Russian star Bivol dominating the weight class, the journey to titles - judging by this showing - still appears challenging.
Ricky Hatton Homage Motivates Local Rivalry Contest
In the supporting bout, Arthur overcame Bradley Rea by divided decision in a outstanding local rivalry battle to secure the continental championship.
The venue grew hushed for a extremely emotional homage ahead of the contest as 10 bells tolled in honor of Manchester's own Hatton, who succumbed 46 years old in recently.
It was the initial significant broadcast fight night in the city since the passing of Hatton. As the last bell faded, the silence became a roar of "there's only one Hatton".
The action inside the ropes equaled the feeling. Arthur was floored by a left hook squarely on the jaw in the early round, standing uncertainly to his position on the count of eight.
But the red-colored equipment of the fighter soon advanced again. Across the ring, Rea - in competitive blue - was marked up early, significant bruising appearing under his face as the fighters landed hard punches.
The challenger, 27, landed power punches but the champion, thirty-four, fought fire with fire with stinging uppercuts as the action hardly diminished over twelve rounds.
The victor edged the result with official scores of draw, narrow lead and 115-112, but the two athletes were persistent and full of heart, all the qualities their hero the legend stood for.
