British light heavyweight Joshua Buatsi earned a win yet underwhelming beginning to life under new promoters with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output seemingly secured him a majority decision in a close ten-round fight at the Manchester venue.
But the challenger landed significant shots, connecting with powerful clean blows in a fight where the styles failed to mesh, with frequent clinching disrupting the action.
The scoring judges had it 96-94 to the winner, with the final judge a 95-95.
"It wasn't my greatest performance but what do champions do? They prevail," the victor commented in the ringside as he was greeted with boos from the spectators.
The Londoner bounced back after his maiden career setback - an epic battle with Callum Smith in last winter - to earn a 20th career victory.
Various analysts thought the challenger, 31, deserved the decision but the contender suffered the second loss of his 28-match career.
The Champion Secures Win in Uneventful Affair
The fighter's advancement has slowed in the past few years through management issues and long spells of inactivity.
Once tipped as the rising talent of UK boxing, he now finds himself trying to achieve expectations - maybe in what appears to be a final roll of the dice - under recent support.
He started carefully, with neither fighter prepared to overcommit in a calculated early stages.
The challenger, whose only previous defeat came at super middleweight against his former opponent in two years ago after suffering a hand injury mid-fight, was participating in his second fight at light-heavyweight.
By the fight's midpoint, the underdog had made it competitive, creating difficulties for Buatsi, who worked behind a solid defense to narrowly win the better stanzas.
The spectators' acceptance decreased during constant clinching, with Parker slipping to the canvas several times during close exchanges.
Parker ended stronger, but several stanzas were hard to judge. Nevertheless, ex-champion the boxing analyst - commentating on Dazn - had it significantly in favor for the challenger, giving Buatsi just a single round.
The victor moves on, but with the likes of power puncher David Benavidez, and Russian star Dmitry Bivol ruling the division, the road to world glory - judging by this showing - still seems difficult.
Ricky Hatton Memorial Inspires Manchester Derby Battle
In the co-main event, Arthur beat Bradley Rea by close verdict in a outstanding Manchester vs Manchester fight to win the continental championship.
The arena fell silent for a deeply poignant tribute ahead of the contest as ten bells rang in remembrance of the city's native Ricky Hatton, who passed away at age 46 in recently.
It was the initial significant broadcast boxing event in the urban center since Hatton's death. As the concluding sound disappeared, the silence transitioned to a roar of "only one Hatton".
The competition within the ring matched the feeling. The eventual winner was floored by a left hook directly to the jaw in the initial stage, rising shakily to his footing on the count of eight.
But the Manchester United-red equipment of the fighter soon advanced again. From the other corner, Rea - in opposing blue - was marked up quickly, significant bruising developing under his orbital area as the fighters traded powerful shots.
Rea, twenty-seven, landed power punches but the champion, thirty-four, fought fire with fire with powerful upward punches as the action hardly diminished over twelve rounds.
The victor barely secured the result with scoring tallies of 114-114, close margin and 115-112, but both fighters were persistent and courageous, all the qualities their hero Hatton stood for.
