UK light heavyweight Joshua Buatsi secured a victory yet underwhelming debut to his career under recent management with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's activity level appeared to earn him a majority decision in a close ten-round bout at the Manchester venue.
But the challenger landed significant shots, delivering effective single shots in a match where the techniques failed to mesh, with constant grabbing disrupting the fight.
The scoring judges scored it 96-94 to the winner, with the third a 95-95.
"It wasn't my best performance but what defines champions? They succeed," Buatsi said in the arena as he was faced negative reactions from the crowd.
The Londoner bounced back after his first professional defeat - an epic battle with Callum Smith in last winter - to secure a twentieth professional win.
Several boxing experts felt Parker, 31, merited the decision but the contender endured the second setback of his 28-fight career.
Buatsi Claims Triumph in Slow Match
His career development has stalled in recent times through promotional disputes and extended periods of lack of fights.
Once tipped as the future of British boxing, he now finds himself trying to fulfil that early promise - maybe in what seems like a final roll of the dice - under recent support.
He opened tentatively, with both competitors willing to be aggressive in a tentative early stages.
Parker, whose sole prior loss came at super-middleweight against John Ryder in two years ago after suffering a hand injury in the contest, was competing in only his second bout at light heavyweight.
By the fight's midpoint, Parker had stayed in contention, causing problems for the favorite, who utilized a tight guard to barely take the more effective periods.
The spectators' acceptance decreased during regular grabbing, with Parker going down various instances during close exchanges.
The underdog finished the stronger, but several stanzas were hard to judge. Nevertheless, ex-champion the boxing analyst - commentating on Dazn - had it particularly wide for Parker, giving Buatsi just only one stanza.
The victor advances, but with the likes of heavy hitter Benavidez, and Eastern European fighter Bivol ruling the division, the path to championship success - on this evidence - still seems difficult.
Ricky Hatton Memorial Fuels Local Rivalry Contest
In the supporting bout, Arthur overcame Rea by split-decision in a excellent Manchester vs Manchester battle to capture the European light heavyweight title.
The arena fell silent for a extremely emotional tribute before the fight as 10 bells tolled in memory of local hero Ricky Hatton, who died 46 years old in recently.
It was the initial significant broadcast match card in the city since Hatton's passing. As the last bell faded, the quiet transitioned to a chant of "there's just one Ricky Hatton".
The action within the ring matched the feeling. Arthur was sent to the canvas by a left hook squarely on the jaw in the early round, getting up unsteadily to his position on the count of eight.
But the Manchester United-red attire of the fighter soon pressed ahead again. On the opposite side, Rea - in competitive blue - was injured soon, significant bruising developing under his face as the duo landed punishing blows.
Rea, twenty-seven, connected with strong blows but the champion, thirty-four, matched the intensity with stinging uppercuts as the pace hardly diminished over the full distance.
Arthur barely secured the result with scoring tallies of draw, narrow lead and wider gap, but each competitor were determined and courageous, exactly what their idol Hatton embodied.
