British light-heavyweight Joshua Buatsi earned a win yet disappointing debut to his career under fresh promotional backing with a points victory over Parker in the Manchester arena.
The Olympic medal winner's punch output appeared to earn him a split decision in a close ten-round fight at the Manchester venue.
But the challenger found success, delivering effective clean blows in a fight where the styles didn't complement each other, with excessive holding disrupting the contest.
The scoring judges had it 96-94 to the winner, with the remaining official a 95-95.
"It wasn't my best display but how do champions respond? They succeed," the victor commented in the ring as he was greeted with negative reactions from the spectators.
The Londoner returned to winning ways after his maiden career setback - an thrilling contest with Callum Smith in last winter - to claim a 20th career victory.
Several boxing experts thought Parker, thirty-one, merited the decision but the Derbyshire fighter endured the second loss of his 28-match professional journey.
The Champion Secures Triumph in Uneventful Match
Buatsi's progress has plateaued in the past few years through management issues and long spells of inactivity.
Once tipped as the next star of UK boxing, he now is positioned trying to achieve expectations - perhaps in what seems like a ultimate opportunity - under recent support.
He opened tentatively, with both competitors ready to take risks in a cagey initial rounds.
The challenger, whose sole prior loss came at super-middleweight against John Ryder in 2022 after damaging his hand mid-fight, was competing in only his second appearance at the higher weight class.
By the fight's midpoint, Parker had stayed in contention, making life awkward for Buatsi, who worked behind a tight guard to barely take the better periods.
The crowd's patience diminished because of frequent holding, with the challenger going down several times during close exchanges.
The underdog finished the stronger, but many rounds were hard to judge. Although, former world champion Carl Frampton - commentating on the broadcast network - had it particularly wide for Parker, scoring for Buatsi just a single round.
The victor advances, but with the caliber of hard hitter Benavidez, and Russian champion Bivol ruling the division, the journey to titles - on this evidence - still looks steep.
Ricky Hatton Homage Motivates Manchester Derby Contest
In the supporting bout, Lyndon Arthur defeated Rea by divided decision in a outstanding local rivalry battle to win the European light-heavyweight title.
The stadium grew hushed for a deeply poignant homage ahead of the contest as 10 bells tolled in memory of Manchester's own Hatton, who passed away aged 46 in last month.
It was the premier televised boxing event in the city since Hatton's passing. As the concluding sound ended, the quiet gave way to a chant of "there's just one Hatton".
The competition inside the ropes reflected the feeling. The eventual winner was sent to the canvas by a left hook directly to the face in the early round, getting up unsteadily to his feet on the eight count.
But the red-colored equipment of the fighter soon pressed ahead again. Across the ring, Rea - in rival City blue - was injured soon, significant bruising forming under his face as the duo exchanged hard punches.
Rea, twenty-seven, landed power punches but the champion, 34, responded in kind with stinging uppercuts as the intensity rarely dropped over twelve rounds.
The victor edged the decision with judges scorecards of draw, 115-113 and wider gap, but the two athletes were relentless and brave, all the qualities their hero the legend embodied.
