UK light heavyweight Joshua Buatsi earned a win yet unimpressive start to life under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic bronze medallist's work rate likely gained him a majority decision in a close 10-round contest at the boxing arena.
But the challenger had his moments, landing some telling clean blows in a match where the approaches didn't complement each other, with excessive holding marring the action.
The officials judged it 96-94 for to Buatsi, with the third a 95-95.
"It wasn't my best showing but what defines champions? They succeed," the victor stated in the ring as he was greeted with negative reactions from the audience.
The Londoner bounced back after his maiden career setback - an epic battle with his previous opponent in February - to secure a 20th pro success.
Some ringside observers felt Parker, thirty-one, should have been awarded the victory but the Derbyshire fighter suffered the second defeat of his 28-bout professional journey.
Buatsi Claims Triumph in Uneventful Encounter
Buatsi's progress has plateaued in recent years through management issues and considerable time of inactivity.
Once tipped as the rising talent of the British fight scene, he now is positioned trying to achieve expectations - possibly in what appears to be a final roll of the dice - under recent support.
He began cautiously, with the two boxers willing to take risks in a tentative early stages.
The challenger, whose only previous defeat came at the lower weight class against his former opponent in two years ago after damaging his hand during the bout, was making just his second appearance at light-heavyweight.
By the middle rounds, the underdog had stayed in contention, making life awkward for Buatsi, who operated with a solid defense to barely take the better rounds.
The spectators' acceptance wore thin amid frequent holding, with Parker going down various instances during grappling.
The underdog closed well, but multiple periods were hard to judge. Although, previous titleholder Carl Frampton - commentating on Dazn - had it strongly leaning for Parker, scoring for Buatsi just only one stanza.
The victor progresses, but with the likes of hard hitter Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the path to championship success - judging by this showing - still looks steep.
Hatton Tribute Fuels Manchester Derby Contest
In the co-main event, Lyndon Arthur defeated Rea by close verdict in a outstanding local rivalry contest to win the continental title.
The stadium became quiet for a deeply poignant homage prior to the bout as 10 bells rang in memory of the city's native Hatton, who passed away aged 46 in last month.
It was the premier televised fight night in the city since Hatton's passing. As the last bell ended, the stillness gave way to a shout of "there's only one Hatton".
The fighting within the ring equaled the sentiment. The eventual winner was floored by a powerful punch directly to the chin in the initial stage, rising shakily to his position on the eight count.
But the red-colored shorts of Arthur soon came forward again. Across the ring, Rea - in competitive blue - was marked up early, significant bruising forming under his eye as the pair landed powerful shots.
The challenger, twenty-seven, landed power punches but the champion, 34, responded in kind with powerful upward punches as the intensity rarely dropped over the full distance.
The victor barely secured the decision with official scores of even score, 115-113 and wider gap, but the two athletes were persistent and courageous, everything their inspiration the legend stood for.
