UK light heavyweight Joshua Buatsi secured a victory yet unimpressive start to life under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic bronze medallist's punch output seemingly secured him a judges' verdict in a tight 10-round fight at the Manchester venue.
But the challenger landed significant shots, connecting with powerful clean blows in a contest where the approaches failed to mesh, with frequent clinching spoiling the action.
The scoring judges judged it 96-94 for to Buatsi, with the third a 95-95.
"It wasn't my best showing but what do champions do? They prevail," the victor commented in the ringside as he was met by negative reactions from the crowd.
The British fighter got back to winning after his initial pro loss - an epic battle with his previous opponent in February - to earn a 20th career victory.
Several boxing experts thought Parker, 31, should have been awarded the victory but the contender endured the second defeat of his 28-match career.
The Champion Secures Win in Slow Affair
The fighter's advancement has stalled in the past few years through management issues and extended periods of inactivity.
Earlier forecast as the rising talent of UK boxing, he now finds himself trying to realize his potential - maybe in what appears to be a final roll of the dice - under recent support.
He started carefully, with neither fighter ready to overcommit in a tentative initial rounds.
The challenger, whose sole prior loss came at the lower weight class against John Ryder in 2022 after damaging his hand in the contest, was competing in only his second fight at the higher weight class.
By the middle rounds, Parker had stayed in contention, creating difficulties for the favorite, who operated with a tight guard to edge the cleaner rounds.
The spectators' acceptance wore thin amid regular grabbing, with Parker falling to the floor various instances during close exchanges.
The underdog closed well, but multiple periods were difficult to score. Nevertheless, ex-champion Carl Frampton - commentating on Dazn - had it particularly wide for Parker, awarding Buatsi just only one stanza.
Buatsi progresses, but with the caliber of power puncher David Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the path to championship success - based on this performance - still appears challenging.
Ricky Hatton Tribute Fuels Manchester Derby Contest
In the co-main event, Arthur beat Rea by split-decision in a excellent local rivalry fight to secure the European light-heavyweight belt.
The venue became quiet for a extremely emotional homage prior to the bout as ten bells rang in honor of local hero Hatton, who succumbed aged 46 in recently.
It was the first major televised match card in the urban center since Hatton's death. As the concluding sound disappeared, the silence transitioned to a roar of "there's only one Hatton".
The action during the bout reflected the feeling. Arthur was knocked down by a significant blow directly to the jaw in the early round, rising shakily to his position on the referee's count.
But the Manchester United-red attire of Arthur soon pressed ahead again. Across the ring, Rea - in rival City blue - was injured early, heavy swelling forming under his face as the duo traded punishing blows.
The challenger, 27, landed power punches but Arthur, 34, responded in kind with powerful upward punches as the pace rarely dropped over the full distance.
The victor narrowly won the verdict with scoring tallies of 114-114, narrow lead and wider gap, but the two athletes were determined and brave, everything their inspiration the legend represented.
