UK light heavyweight Joshua Buatsi secured a victory yet unimpressive debut to life under new promoters with a decision win over Parker in Manchester.
The Olympic bronze medallist's work rate seemingly secured him a split decision in a competitive 10-round fight at the boxing arena.
But Parker landed significant shots, delivering effective individual punches in a match where the techniques failed to mesh, with excessive holding disrupting the fight.
The officials had it 96-94 for to Buatsi, with the final judge a 95-95 draw.
"It wasn't my greatest display but how do champions respond? They prevail," the victor said in the ringside as he was greeted with negative reactions from the spectators.
The Londoner returned to winning ways after his first professional defeat - an thrilling contest with Callum Smith in last winter - to claim a 20th career victory.
Several boxing experts felt Parker, 31, deserved the win but the British boxer experienced the second setback of his 28-bout career.
Buatsi Earns Triumph in Slow Affair
Buatsi's progress has plateaued in recent years through management issues and extended periods of inactivity.
Previously predicted as the rising talent of the British fight scene, he now is positioned trying to fulfil that early promise - possibly in what feels like a ultimate opportunity - under recent support.
He started carefully, with neither fighter prepared to overcommit in a calculated opening few rounds.
Parker, whose single earlier setback came at the lower weight class against his former opponent in two years ago after damaging his hand in the contest, was participating in his second bout at light-heavyweight.
By the fight's midpoint, Parker had stayed in contention, creating difficulties for the favorite, who utilized a tight guard to barely take the more effective periods.
The spectators' acceptance decreased amid constant clinching, with the challenger slipping to the canvas various instances during close exchanges.
Parker closed well, but many rounds were challenging to assess. Although, former world champion Carl Frampton - providing analysis for the broadcast network - had it strongly leaning for the challenger, awarding Buatsi just only one stanza.
Buatsi advances, but with the likes of heavy hitter David Benavidez, and Russian champion Bivol dominating the weight class, the road to world glory - judging by this showing - still appears challenging.
Ricky Hatton Memorial Inspires Local Rivalry Contest
In the co-main event, Lyndon Arthur overcame Rea by split-decision in a excellent local rivalry battle to secure the continental belt.
The venue became quiet for a deeply poignant homage ahead of the contest as ten bells rang in honor of the city's native Ricky Hatton, who passed away at age 46 in recently.
It was the initial significant broadcast match card in the urban center since Hatton's passing. As the final chime ended, the silence became a roar of "there's only one Hatton".
The competition inside the ropes equaled the sentiment. Arthur was floored by a left hook directly to the jaw in the early round, getting up unsteadily to his feet on the count of eight.
But the traditional red equipment of the fighter soon came forward again. Across the ring, the opponent - in opposing blue - was marked up early, heavy swelling forming under his eye as the duo traded powerful shots.
Rea, 27, landed power punches but Arthur, thirty-four, matched the intensity with powerful upward punches as the intensity hardly diminished over twelve rounds.
The victor narrowly won the decision with scoring tallies of 114-114, narrow lead and 115-112, but each competitor were relentless and full of heart, exactly what their inspiration the legend stood for.
