British light-heavyweight Joshua Buatsi secured a victory yet underwhelming beginning to his career under recent management with a points victory over Parker in Manchester.
The Olympic bronze medallist's activity level seemingly secured him a majority decision in a competitive 10-round bout at the Co-op Live Arena.
But the challenger had his moments, connecting with powerful single shots in a fight where the approaches failed to mesh, with excessive holding spoiling the action.
The officials judged it 96-94 for to the winner, with the remaining official a 95-95 draw.
"It wasn't my finest display but how do champions respond? They win," Buatsi commented in the arena as he was faced disapproval from the crowd.
The Londoner bounced back after his maiden career setback - an memorable fight with Callum Smith in last winter - to earn a 20th career victory.
Some ringside observers thought the challenger, thirty-one, should have been awarded the win but the British boxer endured the second setback of his 28-bout professional journey.
The Champion Earns Win in Slow Encounter
The fighter's advancement has plateaued in recent years through contract problems and considerable time of inactivity.
Previously predicted as the next star of British boxing, he now stands trying to achieve expectations - perhaps in what appears to be a final roll of the dice - under fresh management.
He started carefully, with both competitors willing to take risks in a calculated early stages.
The challenger, whose single earlier setback came at super middleweight against his former opponent in two years ago after suffering a hand injury mid-fight, was competing in only his second fight at light heavyweight.
By the halfway stage, the underdog had stayed in contention, causing problems for Buatsi, who utilized a protective stance to narrowly win the better stanzas.
The audience's tolerance decreased during regular grabbing, with the challenger falling to the floor various instances during grappling.
Parker closed well, but several stanzas were hard to judge. Although, former world champion Carl Frampton - commentating on the broadcast network - had it particularly wide for Parker, awarding Buatsi just only one stanza.
The victor progresses, but with the likes of hard hitter Benavidez, and Russian champion Bivol leading the category, the journey to titles - based on this performance - still seems difficult.
Ricky Hatton Memorial Inspires Local Rivalry Battle
In the supporting bout, Lyndon Arthur overcame Bradley Rea by close verdict in a outstanding Manchester vs Manchester contest to win the continental championship.
The arena grew hushed for a deeply poignant homage prior to the bout as 10 bells rang in memory of local hero Ricky Hatton, who passed away 46 years old in last month.
It was the first major televised fight night in the urban center since Hatton's passing. As the final chime ended, the quiet gave way to a chant of "there's only one Ricky Hatton".
The competition within the ring equaled the emotion. Arthur was floored by a powerful punch directly to the jaw in the second round, standing uncertainly to his position on the referee's count.
But the red-colored equipment of Arthur soon advanced again. On the opposite side, Rea - in opposing blue - was damaged quickly, significant bruising forming under his orbital area as the pair exchanged hard punches.
Rea, 27, landed power punches but Arthur, 34, responded in kind with effective uppercuts as the pace rarely dropped over the full distance.
The victor edged the verdict with scoring tallies of even score, 115-113 and clearer margin, but both fighters were relentless and full of heart, exactly what their idol Hatton represented.
