UK light-heavyweight Joshua Buatsi secured a victory yet unimpressive beginning to life under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic bronze medallist's punch output seemingly secured him a judges' verdict in a close ten-round fight at the Manchester venue.
But the challenger landed significant shots, connecting with powerful individual punches in a contest where the approaches didn't complement each other, with excessive holding marring the fight.
Two judges scored it 96-94 in favor to Buatsi, with the remaining official a 95-95 draw.
"It wasn't my finest display but how do champions respond? They succeed," Buatsi commented in the arena as he was faced negative reactions from the audience.
The Londoner returned to winning ways after his first professional defeat - an epic battle with Callum Smith in last winter - to secure a 20th pro success.
Various analysts felt Parker, 31, merited the decision but the contender suffered the second loss of his 28-fight professional journey.
The Champion Claims Win in Dull Affair
Buatsi's progress has plateaued in recent times through contract problems and long spells of lack of fights.
Earlier forecast as the rising talent of British boxing, he now finds himself trying to fulfil that early promise - maybe in what appears to be a ultimate opportunity - under fresh management.
He started carefully, with both competitors ready to be aggressive in a cagey early stages.
Parker, whose only previous defeat came at super middleweight against John Ryder in two years ago after suffering a hand injury mid-fight, was competing in only his second bout at light heavyweight.
By the halfway stage, Parker had stayed in contention, causing problems for Buatsi, who utilized a tight guard to narrowly win the better periods.
The spectators' acceptance wore thin amid frequent holding, with Parker going down several times during clinches.
The underdog finished the stronger, but many rounds were hard to judge. Although, previous titleholder Carl Frampton - working with the broadcast network - had it strongly leaning for Parker, giving Buatsi just only one stanza.
The victor advances, but with the caliber of power puncher David Benavidez, and Russian star Bivol dominating the weight class, the path to championship success - based on this performance - still appears challenging.
Hatton Memorial Motivates Local Rivalry Battle
In the supporting bout, Arthur beat Rea by divided decision in a terrific Manchester vs Manchester contest to secure the European light-heavyweight belt.
The arena grew hushed for a profoundly moving homage before the fight as 10 bells tolled in remembrance of Manchester's own Ricky Hatton, who passed away at age 46 in September.
It was the premier televised boxing event in the location since the passing of Hatton. As the final chime disappeared, the stillness became a chant of "only one Ricky Hatton".
The fighting within the ring matched the emotion. The eventual winner was sent to the canvas by a significant blow flush on the jaw in the second round, getting up unsteadily to his footing on the eight count.
But the traditional red equipment of Arthur soon advanced again. From the other corner, Rea - in rival City blue - was marked up quickly, heavy swelling appearing under his orbital area as the fighters traded punishing blows.
The challenger, 27, landed power punches but the champion, 34, matched the intensity with stinging uppercuts as the pace seldom decreased over the full distance.
The victor edged the result with scoring tallies of even score, close margin and 115-112, but both fighters were persistent and courageous, exactly what their inspiration Hatton represented.
