UK light-heavyweight Joshua Buatsi secured a victory yet unimpressive beginning to life under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic medal winner's work rate appeared to earn him a majority decision in a close 10-round bout at the Manchester venue.
But Parker landed significant shots, landing some telling clean blows in a match where the techniques never quite clicked, with constant grabbing spoiling the fight.
The scoring judges judged it 96-94 in favor to Buatsi, with the final judge a even score.
"It wasn't my best display but how do champions respond? They win," Buatsi stated in the ring as he was met by boos from the crowd.
The British fighter got back to winning after his maiden career setback - an epic battle with his previous opponent in last winter - to earn a twentieth professional win.
Several boxing experts felt the challenger, 31, deserved the victory but the British boxer endured the second loss of his 28-bout career.
The Champion Claims Victory in Dull Match
The fighter's advancement has stalled in recent years through contract problems and long spells of lack of fights.
Once tipped as the rising talent of British boxing, he now finds himself trying to realize his potential - possibly in what seems like a ultimate opportunity - under new promotional backing.
He began cautiously, with the two boxers willing to be aggressive in a calculated early stages.
Parker, whose single earlier setback came at super middleweight against John Ryder in two years ago after breaking his hand during the bout, was making just his second bout at the higher weight class.
By the fight's midpoint, Parker had kept it close, causing problems for the favorite, who worked behind a solid defense to barely take the better rounds.
The spectators' acceptance decreased amid regular grabbing, with Parker falling to the floor multiple occasions during close exchanges.
The underdog ended stronger, but several stanzas were difficult to score. However, previous titleholder Carl Frampton - providing analysis for Dazn - had it strongly leaning for Parker, awarding Buatsi just a single round.
The victor moves on, but with the likes of hard hitter Benavidez, and Russian champion Dmitry Bivol leading the category, the road to world glory - on this evidence - still seems difficult.
Hatton Memorial Inspires All-Manchester Fight
In the co-main event, Arthur overcame Bradley Rea by close verdict in a terrific Manchester vs Manchester battle to capture the European light-heavyweight championship.
The arena grew hushed for a profoundly moving homage ahead of the contest as a bell ceremony rang in honor of Manchester's own Ricky Hatton, who succumbed 46 years old in September.
It was the first major televised fight night in the urban center since Hatton's passing. As the final chime faded, the silence gave way to a roar of "only one Ricky Hatton".
The competition within the ring equaled the emotion. Arthur was floored by a powerful punch squarely on the face in the early round, getting up unsteadily to his position on the referee's count.
But the red-colored attire of Arthur soon advanced again. On the opposite side, Rea - in competitive blue - was damaged soon, noticeable damage appearing under his eye as the fighters exchanged hard punches.
Rea, twenty-seven, landed power punches but Arthur, thirty-four, responded in kind with powerful upward punches as the intensity seldom decreased over the full distance.
Arthur barely secured the decision with scoring tallies of even score, narrow lead and clearer margin, but the two athletes were determined and full of heart, exactly what their inspiration the legend stood for.
