UK light heavyweight Joshua Buatsi earned a win yet disappointing beginning to life under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic medal winner's punch output appeared to earn him a judges' verdict in a close ten-round fight at the Co-op Live Arena.
But the challenger found success, connecting with powerful single shots in a fight where the styles failed to mesh, with constant grabbing spoiling the contest.
Two judges had it 96-94 for to the winner, with the third a 95-95 draw.
"It wasn't my best display but what defines champions? They win," the victor stated in the ringside as he was faced negative reactions from the audience.
The Londoner returned to winning ways after his first professional defeat - an epic battle with his previous opponent in last winter - to claim a 20th pro success.
Some ringside observers thought the challenger, 31, should have been awarded the win but the Derbyshire fighter endured the second setback of his 28-fight career.
Buatsi Secures Triumph in Slow Affair
His career development has slowed in recent years through promotional disputes and long spells of lack of fights.
Once tipped as the next star of British boxing, he now is positioned trying to realize his potential - maybe in what feels like a final roll of the dice - under recent support.
He started carefully, with neither fighter ready to take risks in a tentative initial rounds.
The challenger, whose sole prior loss came at the lower weight class against John Ryder in 2022 after suffering a hand injury during the bout, was participating in his second appearance at light heavyweight.
By the halfway stage, the underdog had stayed in contention, making life awkward for the favorite, who worked behind a tight guard to narrowly win the better periods.
The audience's tolerance diminished during frequent holding, with Parker slipping to the canvas several times during close exchanges.
The underdog finished the stronger, but multiple periods were challenging to assess. However, previous titleholder the boxing analyst - providing analysis for Dazn - had it significantly in favor for the challenger, awarding Buatsi just one round.
Buatsi moves on, but with the presence of hard hitter David Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the journey to titles - judging by this showing - still seems difficult.
Hatton Tribute Motivates Local Rivalry Contest
In the secondary fight, Arthur beat Rea by close verdict in a terrific local rivalry contest to secure the continental belt.
The stadium fell silent for a extremely emotional homage ahead of the contest as ten bells rang in honor of Manchester's own Hatton, who passed away 46 years old in September.
It was the first major televised match card in the location since Hatton's death. As the final chime disappeared, the stillness became a roar of "only one Ricky Hatton".
The action inside the ropes reflected the sentiment. Arthur was floored by a significant blow directly to the jaw in the initial stage, getting up unsteadily to his footing on the eight count.
But the red-colored equipment of Arthur soon advanced again. On the opposite side, Rea - in rival City blue - was injured quickly, heavy swelling appearing under his eye as the pair landed punishing blows.
The challenger, 27, landed power punches but Arthur, 34, matched the intensity with effective uppercuts as the pace rarely dropped over the full distance.
Arthur narrowly won the verdict with official scores of even score, narrow lead and wider gap, but each competitor were persistent and full of heart, everything their idol the legend embodied.
