UK light heavyweight Joshua Buatsi earned a win yet unimpressive start to his career under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's activity level appeared to earn him a judges' verdict in a tight 10-round fight at the boxing arena.
But the challenger had his moments, connecting with powerful single shots in a contest where the techniques didn't complement each other, with excessive holding marring the action.
Two judges scored it 96-94 to Buatsi, with the remaining official a 95-95 draw.
"It wasn't my greatest showing but how do champions respond? They succeed," Buatsi commented in the ringside as he was met by disapproval from the spectators.
The Londoner returned to winning ways after his initial pro loss - an thrilling contest with Callum Smith in last winter - to earn a 20th career victory.
Various analysts thought the challenger, thirty-one, should have been awarded the victory but the contender suffered the second setback of his 28-fight career.
The Champion Earns Win in Slow Affair
Buatsi's progress has stalled in recent times through promotional disputes and considerable time of lack of fights.
Earlier forecast as the rising talent of British boxing, he now is positioned trying to realize his potential - maybe in what feels like a ultimate opportunity - under fresh management.
He began cautiously, with neither fighter willing to take risks in a cagey early stages.
Parker, whose sole prior loss came at super-middleweight against his former opponent in 2022 after damaging his hand during the bout, was participating in his second appearance at the higher weight class.
By the halfway stage, Parker had made it competitive, causing problems for the favorite, who worked behind a solid defense to narrowly win the better stanzas.
The crowd's patience decreased during regular grabbing, with Parker slipping to the canvas several times during clinches.
The underdog finished the stronger, but multiple periods were difficult to score. Although, former world champion Carl Frampton - working with Dazn - had it strongly leaning for Parker, awarding Buatsi just one round.
Buatsi progresses, but with the caliber of power puncher Benavidez, and Russian star Bivol dominating the weight class, the path to championship success - based on this performance - still seems difficult.
Ricky Hatton Tribute Motivates Manchester Derby Fight
In the supporting bout, Arthur defeated Rea by close verdict in a outstanding Manchester vs Manchester fight to capture the European light-heavyweight belt.
The arena became quiet for a extremely emotional homage ahead of the contest as 10 bells tolled in memory of Manchester's own Ricky Hatton, who died at age 46 in last month.
It was the first major televised match card in the city since Hatton's death. As the concluding sound faded, the stillness transitioned to a roar of "there's just one Ricky Hatton".
The fighting during the bout reflected the feeling. The eventual winner was floored by a significant blow flush on the face in the early round, rising shakily to his footing on the eight count.
But the Manchester United-red attire of Arthur soon came forward again. Across the ring, Rea - in competitive blue - was damaged quickly, heavy swelling appearing under his orbital area as the pair exchanged hard punches.
Rea, twenty-seven, connected with strong blows but Arthur, 34, responded in kind with effective uppercuts as the action hardly diminished over twelve rounds.
Arthur barely secured the decision with scoring tallies of 114-114, narrow lead and clearer margin, but each competitor were relentless and brave, everything their inspiration the legend embodied.
