UK light heavyweight Joshua Buatsi secured a victory yet disappointing start to life under recent management with a points victory over Parker in the Manchester arena.
The Olympic medal winner's activity level appeared to earn him a judges' verdict in a tight ten-round bout at the Manchester venue.
But the challenger found success, delivering effective individual punches in a contest where the approaches never quite clicked, with excessive holding disrupting the fight.
Two judges judged it 96-94 in favor to Buatsi, with the final judge a even score.
"It wasn't my finest performance but what do champions do? They succeed," the victor stated in the ring as he was faced boos from the spectators.
The Londoner bounced back after his initial pro loss - an thrilling contest with Callum Smith in February - to earn a 20th pro success.
Several boxing experts thought Parker, thirty-one, deserved the win but the British boxer suffered the second setback of his 28-match professional journey.
The Champion Earns Win in Uneventful Match
Buatsi's progress has slowed in recent years through contract problems and extended periods of inactivity.
Previously predicted as the next star of UK boxing, he now stands trying to realize his potential - perhaps in what seems like a last chance - under fresh management.
He opened tentatively, with the two boxers ready to be aggressive in a calculated early stages.
The challenger, whose single earlier setback came at super middleweight against his former opponent in 2022 after damaging his hand during the bout, was making just his second appearance at the higher weight class.
By the halfway stage, the underdog had made it competitive, creating difficulties for Buatsi, who worked behind a protective stance to barely take the better rounds.
The audience's tolerance diminished amid frequent holding, with the challenger slipping to the canvas several times during grappling.
Parker finished the stronger, but many rounds were difficult to score. Nevertheless, previous titleholder Carl Frampton - working with Dazn - had it particularly wide for Parker, scoring for Buatsi just only one stanza.
The victor moves on, but with the likes of power puncher Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the road to world glory - on this evidence - still appears challenging.
Ricky Hatton Homage Inspires Local Rivalry Battle
In the secondary fight, Arthur defeated Rea by close verdict in a outstanding local rivalry battle to secure the European light-heavyweight belt.
The stadium grew hushed for a profoundly moving memorial prior to the bout as 10 bells rang in remembrance of Manchester's own Ricky Hatton, who succumbed at age 46 in recently.
It was the initial significant broadcast fight night in the city since Hatton's death. As the final chime disappeared, the stillness gave way to a shout of "there's just one Hatton".
The action inside the ropes matched the emotion. The eventual winner was floored by a powerful punch directly to the chin in the initial stage, getting up unsteadily to his position on the count of eight.
But the traditional red shorts of the fighter soon pressed ahead again. From the other corner, the opponent - in competitive blue - was damaged quickly, significant bruising appearing under his eye as the pair landed punishing blows.
Rea, 27, landed power punches but the champion, 34, fought fire with fire with powerful upward punches as the action hardly diminished over 12 rounds.
The victor narrowly won the decision with scoring tallies of 114-114, 115-113 and clearer margin, but the two athletes were relentless and courageous, everything their inspiration the legend embodied.
