UK light heavyweight Joshua Buatsi earned a win yet underwhelming debut to life under new promoters with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output likely gained him a majority decision in a close 10-round contest at the boxing arena.
But Parker landed significant shots, delivering effective clean blows in a match where the styles failed to mesh, with constant grabbing disrupting the action.
Two judges judged it 96-94 to the winner, with the remaining official a 95-95 draw.
"It wasn't my greatest showing but how do champions respond? They succeed," Buatsi stated in the ringside as he was greeted with disapproval from the audience.
The Londoner bounced back after his maiden career setback - an thrilling contest with his previous opponent in last winter - to claim a twentieth professional win.
Various analysts felt Parker, 31, merited the win but the Derbyshire fighter suffered the second setback of his 28-bout career.
The Champion Earns Victory in Uneventful Encounter
His career development has stalled in recent times through contract problems and extended periods of inactivity.
Once tipped as the next star of British boxing, he now finds himself trying to realize his potential - perhaps in what seems like a last chance - under new promotional backing.
He opened tentatively, with neither fighter willing to take risks in a calculated opening few rounds.
The challenger, whose only previous defeat came at the lower weight class against John Ryder in 2022 after damaging his hand in the contest, was participating in his second appearance at the higher weight class.
By the middle rounds, Parker had made it competitive, creating difficulties for the favorite, who operated with a protective stance to edge the cleaner periods.
The spectators' acceptance decreased because of regular grabbing, with the challenger going down various instances during close exchanges.
The underdog finished the stronger, but several stanzas were difficult to score. Although, ex-champion Carl Frampton - commentating on Dazn - had it particularly wide for the challenger, giving Buatsi just one round.
The victor progresses, but with the likes of heavy hitter David Benavidez, and Russian champion Bivol leading the category, the path to championship success - on this evidence - still looks steep.
Ricky Hatton Homage Motivates Manchester Derby Fight
In the secondary fight, Arthur overcame Rea by split-decision in a excellent Manchester vs Manchester fight to win the European light-heavyweight belt.
The venue became quiet for a profoundly moving tribute ahead of the contest as a bell ceremony tolled in memory of local hero Hatton, who died 46 years old in last month.
It was the initial significant broadcast match card in the location since Hatton's death. As the concluding sound ended, the quiet transitioned to a shout of "there's only one Ricky Hatton".
The action inside the ropes equaled the feeling. The eventual winner was knocked down by a powerful punch squarely on the chin in the second round, getting up unsteadily to his footing on the count of eight.
But the traditional red shorts of Arthur soon pressed ahead again. From the other corner, the opponent - in opposing blue - was injured quickly, heavy swelling developing under his face as the fighters landed punishing blows.
The challenger, twenty-seven, landed power punches but the champion, 34, fought fire with fire with stinging uppercuts as the pace rarely dropped over 12 rounds.
The victor barely secured the verdict with judges scorecards of 114-114, close margin and wider gap, but each competitor were determined and full of heart, everything their hero Hatton embodied.
