British light heavyweight Joshua Buatsi earned a win yet disappointing debut to life under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's activity level appeared to earn him a split decision in a tight 10-round fight at the Co-op Live Arena.
But Parker landed significant shots, delivering effective clean blows in a match where the techniques didn't complement each other, with excessive holding spoiling the fight.
The scoring judges judged it 96-94 to Buatsi, with the remaining official a 95-95.
"It wasn't my greatest performance but what do champions do? They succeed," Buatsi commented in the ringside as he was faced disapproval from the spectators.
The Londoner returned to winning ways after his maiden career setback - an epic battle with Callum Smith in February - to earn a 20th career victory.
Various analysts thought Parker, 31, merited the win but the contender experienced the second defeat of his 28-match career.
The Champion Earns Win in Dull Encounter
Buatsi's progress has stalled in recent times through contract problems and long spells of inactivity.
Previously predicted as the next star of the British fight scene, he now finds himself trying to achieve expectations - maybe in what feels like a last chance - under new promotional backing.
He started carefully, with the two boxers prepared to overcommit in a cagey opening few rounds.
The challenger, whose single earlier setback came at the lower weight class against John Ryder in two years ago after damaging his hand during the bout, was making just his second fight at light heavyweight.
By the fight's midpoint, Parker had stayed in contention, making life awkward for Buatsi, who worked behind a solid defense to edge the more effective rounds.
The crowd's patience diminished because of regular grabbing, with Parker slipping to the canvas multiple occasions during clinches.
Parker closed well, but multiple periods were difficult to score. Although, previous titleholder Carl Frampton - working with Dazn - had it significantly in favor for Parker, giving Buatsi just one round.
The victor advances, but with the likes of power puncher Benavidez, and Eastern European fighter Bivol dominating the weight class, the road to world glory - based on this performance - still looks steep.
Hatton Tribute Inspires All-Manchester Battle
In the secondary fight, Arthur overcame Rea by close verdict in a outstanding local rivalry battle to win the European light-heavyweight title.
The arena fell silent for a deeply poignant homage ahead of the contest as a bell ceremony sounded in remembrance of the city's native Hatton, who passed away 46 years old in September.
It was the premier televised boxing event in the city since the passing of Hatton. As the final chime ended, the silence became a shout of "only one Ricky Hatton".
The action within the ring reflected the sentiment. The eventual winner was floored by a left hook squarely on the jaw in the initial stage, getting up unsteadily to his position on the count of eight.
But the red-colored equipment of Arthur soon pressed ahead again. From the other corner, the opponent - in competitive blue - was injured early, heavy swelling developing under his eye as the pair traded hard punches.
The challenger, twenty-seven, connected with strong blows but Arthur, 34, matched the intensity with effective uppercuts as the pace hardly diminished over 12 rounds.
The victor edged the result with official scores of even score, narrow lead and wider gap, but both fighters were relentless and courageous, everything their hero the legend represented.
