British light-heavyweight Joshua Buatsi made a winning yet underwhelming debut to his career under new promoters with a decision win over Parker in Manchester.
The Olympic medal winner's punch output appeared to earn him a judges' verdict in a tight 10-round bout at the Manchester venue.
But Parker had his moments, delivering effective individual punches in a match where the styles didn't complement each other, with excessive holding disrupting the fight.
The officials judged it 96-94 to the winner, with the final judge a even score.
"It wasn't my greatest showing but what defines champions? They prevail," the victor said in the ring as he was faced disapproval from the spectators.
The Londoner returned to winning ways after his maiden career setback - an thrilling contest with Callum Smith in last winter - to secure a twentieth professional win.
Various analysts felt Parker, 31, should have been awarded the victory but the British boxer suffered the second defeat of his 28-bout career.
The Champion Earns Triumph in Slow Encounter
Buatsi's progress has slowed in the past few years through promotional disputes and long spells of inactivity.
Previously predicted as the next star of British boxing, he now is positioned trying to realize his potential - possibly in what seems like a last chance - under fresh management.
He started carefully, with both competitors ready to overcommit in a cagey initial rounds.
Parker, whose sole prior loss came at super-middleweight against his former opponent in 2022 after breaking his hand in the contest, was making just his second appearance at the higher weight class.
By the fight's midpoint, the underdog had kept it close, creating difficulties for Buatsi, who operated with a solid defense to narrowly win the more effective periods.
The crowd's patience wore thin because of frequent holding, with the challenger going down various instances during clinches.
Parker closed well, but many rounds were difficult to score. Nevertheless, ex-champion Carl Frampton - providing analysis for the broadcast network - had it particularly wide for the challenger, scoring for Buatsi just one round.
Buatsi moves on, but with the caliber of heavy hitter Benavidez, and Russian star Bivol dominating the weight class, the journey to titles - based on this performance - still looks steep.
Hatton Memorial Inspires Local Rivalry Contest
In the co-main event, Lyndon Arthur defeated Rea by split-decision in a excellent local rivalry fight to capture the European light-heavyweight title.
The stadium fell silent for a profoundly moving homage prior to the bout as 10 bells rang in honor of the city's native Hatton, who passed away aged 46 in last month.
It was the initial significant broadcast match card in the location since Hatton's passing. As the concluding sound faded, the silence transitioned to a roar of "there's just one Ricky Hatton".
The competition inside the ropes matched the sentiment. Arthur was knocked down by a significant blow flush on the jaw in the initial stage, standing uncertainly to his footing on the eight count.
But the red-colored shorts of Arthur soon came forward again. Across the ring, Rea - in rival City blue - was injured soon, significant bruising forming under his face as the fighters exchanged punishing blows.
The challenger, 27, landed power punches but the champion, thirty-four, fought fire with fire with powerful upward punches as the pace seldom decreased over twelve rounds.
The victor edged the verdict with judges scorecards of 114-114, close margin and clearer margin, but both fighters were relentless and full of heart, everything their hero Hatton embodied.
