UK light heavyweight Joshua Buatsi made a winning yet underwhelming debut to life under recent management with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's activity level appeared to earn him a majority decision in a competitive ten-round fight at the Co-op Live Arena.
But Parker had his moments, connecting with powerful single shots in a match where the styles never quite clicked, with frequent clinching spoiling the action.
The scoring judges had it 96-94 in favor to Buatsi, with the third a even score.
"It wasn't my best performance but what defines champions? They prevail," the victor said in the ring as he was faced negative reactions from the audience.
The Londoner got back to winning after his initial pro loss - an memorable fight with his previous opponent in last winter - to secure a twentieth professional win.
Several boxing experts believed the challenger, thirty-one, should have been awarded the decision but the contender experienced the second loss of his 28-bout professional journey.
The Champion Claims Victory in Slow Match
The fighter's advancement has plateaued in recent times through promotional disputes and considerable time of inactivity.
Earlier forecast as the rising talent of UK boxing, he now is positioned trying to achieve expectations - possibly in what seems like a ultimate opportunity - under new promotional backing.
He started carefully, with the two boxers ready to be aggressive in a cagey early stages.
Parker, whose only previous defeat came at the lower weight class against John Ryder in two years ago after suffering a hand injury in the contest, was participating in his second fight at light heavyweight.
By the halfway stage, Parker had made it competitive, making life awkward for the favorite, who utilized a tight guard to barely take the more effective periods.
The audience's tolerance diminished because of frequent holding, with the challenger falling to the floor several times during clinches.
The underdog ended stronger, but multiple periods were difficult to score. However, ex-champion Carl Frampton - commentating on the broadcast network - had it particularly wide for Parker, scoring for Buatsi just one round.
Buatsi advances, but with the likes of hard hitter David Benavidez, and Eastern European fighter Bivol leading the category, the journey to titles - based on this performance - still looks steep.
Hatton Tribute Fuels Manchester Derby Fight
In the supporting bout, Arthur defeated Rea by divided decision in a terrific local rivalry fight to win the European light-heavyweight title.
The arena became quiet for a profoundly moving tribute before the fight as a bell ceremony tolled in honor of the city's native Ricky Hatton, who succumbed 46 years old in last month.
It was the initial significant broadcast match card in the urban center since the passing of Hatton. As the final chime disappeared, the stillness gave way to a chant of "there's only one Hatton".
The action within the ring equaled the sentiment. The eventual winner was knocked down by a significant blow directly to the chin in the early round, standing uncertainly to his position on the eight count.
But the traditional red equipment of Arthur soon pressed ahead again. On the opposite side, Rea - in competitive blue - was injured quickly, noticeable damage developing under his orbital area as the duo exchanged punishing blows.
The challenger, twenty-seven, landed power punches but Arthur, 34, responded in kind with powerful upward punches as the intensity hardly diminished over 12 rounds.
The victor edged the result with scoring tallies of even score, 115-113 and 115-112, but each competitor were relentless and brave, everything their inspiration the legend embodied.
