British light heavyweight Joshua Buatsi earned a win yet underwhelming beginning to life under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's work rate seemingly secured him a split decision in a tight ten-round bout at the boxing arena.
But Parker found success, landing some telling individual punches in a contest where the techniques never quite clicked, with constant grabbing marring the fight.
The scoring judges scored it 96-94 in favor to Buatsi, with the final judge a even score.
"It wasn't my greatest display but how do champions respond? They win," Buatsi said in the arena as he was faced disapproval from the spectators.
The Londoner bounced back after his first professional defeat - an thrilling contest with Callum Smith in February - to earn a 20th career victory.
Various analysts believed Parker, thirty-one, should have been awarded the decision but the contender experienced the second defeat of his 28-match career.
Buatsi Secures Triumph in Slow Match
Buatsi's progress has stalled in the past few years through management issues and long spells of lack of fights.
Previously predicted as the next star of the British fight scene, he now stands trying to realize his potential - perhaps in what feels like a final roll of the dice - under fresh management.
He started carefully, with the two boxers ready to be aggressive in a cagey opening few rounds.
The challenger, whose single earlier setback came at super middleweight against his former opponent in two years ago after suffering a hand injury during the bout, was competing in only his second fight at the higher weight class.
By the fight's midpoint, Parker had made it competitive, creating difficulties for the favorite, who utilized a tight guard to edge the more effective periods.
The crowd's patience decreased because of constant clinching, with Parker falling to the floor various instances during clinches.
Parker ended stronger, but multiple periods were challenging to assess. However, ex-champion the boxing analyst - providing analysis for Dazn - had it significantly in favor for Parker, scoring for Buatsi just a single round.
Buatsi progresses, but with the presence of heavy hitter Benavidez, and Russian champion Dmitry Bivol ruling the division, the path to championship success - on this evidence - still appears challenging.
Hatton Memorial Fuels Manchester Derby Contest
In the supporting bout, Arthur beat Rea by close verdict in a outstanding Manchester vs Manchester contest to secure the continental belt.
The arena grew hushed for a profoundly moving tribute ahead of the contest as a bell ceremony sounded in memory of local hero Hatton, who died 46 years old in September.
It was the first major televised boxing event in the location since the passing of Hatton. As the concluding sound faded, the quiet gave way to a shout of "there's just one Ricky Hatton".
The competition within the ring matched the feeling. Arthur was knocked down by a significant blow flush on the jaw in the early round, getting up unsteadily to his footing on the referee's count.
But the Manchester United-red shorts of the fighter soon pressed ahead again. On the opposite side, the opponent - in opposing blue - was damaged soon, noticeable damage appearing under his orbital area as the fighters traded hard punches.
The challenger, twenty-seven, landed power punches but the champion, thirty-four, matched the intensity with powerful upward punches as the intensity seldom decreased over twelve rounds.
Arthur edged the verdict with judges scorecards of draw, narrow lead and 115-112, but each competitor were persistent and courageous, everything their inspiration the legend stood for.
