UK light-heavyweight Joshua Buatsi earned a win yet unimpressive start to life under new promoters with a points victory over Parker in Manchester.
The Olympic bronze medallist's punch output likely gained him a judges' verdict in a tight ten-round fight at the Manchester venue.
But the challenger landed significant shots, landing some telling single shots in a fight where the styles failed to mesh, with constant grabbing marring the action.
The officials judged it 96-94 for to the winner, with the third a 95-95.
"It wasn't my greatest showing but how do champions respond? They prevail," the victor said in the arena as he was faced disapproval from the spectators.
The British fighter bounced back after his initial pro loss - an thrilling contest with Callum Smith in last winter - to claim a twentieth professional win.
Several boxing experts thought the challenger, thirty-one, merited the win but the British boxer endured the second setback of his 28-fight professional journey.
The Champion Earns Victory in Uneventful Affair
The fighter's advancement has stalled in recent years through management issues and considerable time of inactivity.
Earlier forecast as the future of UK 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 opened tentatively, with the two boxers prepared to overcommit in a tentative initial rounds.
Parker, whose sole prior loss came at super-middleweight against his former opponent in 2022 after damaging his hand in the contest, was making just his second bout at light heavyweight.
By the middle rounds, the underdog had stayed in contention, causing problems for Buatsi, who worked behind a solid defense to barely take the better rounds.
The spectators' acceptance decreased during regular grabbing, with Parker falling to the floor various instances during close exchanges.
The underdog closed well, but several stanzas were difficult to score. Nevertheless, previous titleholder the boxing analyst - commentating on Dazn - had it significantly in favor for Parker, scoring for Buatsi just one round.
The victor moves on, but with the likes of hard hitter Benavidez, and Russian star Bivol dominating the weight class, the path to championship success - on this evidence - still appears challenging.
Ricky Hatton Tribute Motivates Local Rivalry Battle
In the supporting bout, Arthur overcame Rea by close verdict in a terrific all-Manchester contest to capture the European light heavyweight belt.
The arena fell silent for a extremely emotional homage before the fight as ten bells sounded in memory of the city's native Ricky Hatton, who succumbed aged 46 in recently.
It was the initial significant broadcast fight night in the city since the passing of Hatton. As the last bell faded, the silence gave way to a roar of "there's just one Ricky Hatton".
The action inside the ropes equaled the emotion. The eventual winner was knocked down by a left hook directly to the chin in the initial stage, rising shakily to his footing on the count of eight.
But the red-colored attire of the fighter soon came forward again. On the opposite side, Rea - in opposing blue - was damaged quickly, noticeable damage forming under his orbital area as the duo landed punishing blows.
Rea, 27, landed power punches but the champion, thirty-four, matched the intensity with effective uppercuts as the action seldom decreased over 12 rounds.
Arthur barely secured the decision with judges scorecards of 114-114, close margin and wider gap, but the two athletes were determined and courageous, exactly what their inspiration Hatton embodied.
