UK light heavyweight Joshua Buatsi earned a win yet disappointing beginning to life under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's work rate appeared to earn him a majority decision in a competitive ten-round fight at the Co-op Live Arena.
But the challenger landed significant shots, connecting with powerful individual punches in a match where the techniques failed to mesh, with excessive holding disrupting the fight.
The scoring judges had it 96-94 to Buatsi, with the third a 95-95.
"It wasn't my best display but what defines champions? They win," the victor said in the ring as he was greeted with negative reactions from the audience.
The British fighter bounced back after his first professional defeat - an thrilling contest with his previous opponent in last winter - to secure a 20th career victory.
Some ringside observers felt the challenger, thirty-one, merited the win but the British boxer endured the second setback of his 28-bout career.
Buatsi Claims Win in Slow Affair
Buatsi's progress has plateaued in recent years through management issues and long spells of lack of fights.
Once tipped as the next star of British boxing, he now finds himself trying to achieve expectations - maybe in what feels like a ultimate opportunity - under new promotional backing.
He began cautiously, with both competitors ready to take risks in a cagey initial rounds.
Parker, whose single earlier setback came at the lower weight class against his former opponent in two years ago after suffering a hand injury mid-fight, was participating in his second appearance at light heavyweight.
By the halfway stage, the underdog had stayed in contention, making life awkward for Buatsi, who utilized a solid defense to barely take the cleaner rounds.
The crowd's patience wore thin during constant clinching, with Parker slipping to the canvas multiple occasions during close exchanges.
Parker finished the stronger, but several stanzas were challenging to assess. Although, former world champion the boxing analyst - working with the broadcast network - had it strongly leaning for the challenger, awarding Buatsi just a single round.
Buatsi moves on, but with the caliber of heavy hitter David Benavidez, and Eastern European fighter Bivol dominating the weight class, the journey to titles - judging by this showing - still seems difficult.
Hatton Memorial Inspires Manchester Derby Contest
In the secondary fight, Lyndon Arthur beat Rea by split-decision in a outstanding local rivalry contest to capture the continental belt.
The stadium became quiet for a profoundly moving tribute before the fight as 10 bells rang in memory of the city's native Hatton, who succumbed aged 46 in recently.
It was the premier televised boxing event in the location since the passing of Hatton. As the final chime disappeared, the stillness transitioned to a shout of "there's only one Hatton".
The action during the bout reflected the feeling. Arthur was knocked down by a significant blow squarely on the chin in the initial stage, rising shakily to his footing on the eight count.
But the Manchester United-red shorts of the fighter soon advanced again. Across the ring, Rea - in opposing blue - was marked up early, noticeable damage forming under his orbital area as the duo exchanged punishing blows.
Rea, twenty-seven, landed power punches but Arthur, 34, matched the intensity with effective uppercuts as the pace seldom decreased over twelve rounds.
The victor edged the verdict with scoring tallies of draw, narrow lead and 115-112, but both fighters were relentless and brave, everything their hero Hatton stood for.
