UK light heavyweight Joshua Buatsi made a winning yet disappointing debut to life under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's work rate appeared to earn him a majority decision in a tight 10-round bout at the Co-op Live Arena.
But Parker landed significant shots, connecting with powerful clean blows in a contest where the approaches failed to mesh, with constant grabbing spoiling the action.
The scoring judges had it 96-94 to Buatsi, with the remaining official a 95-95.
"It wasn't my best performance but what do champions do? They succeed," Buatsi stated in the ringside as he was faced disapproval from the crowd.
The British fighter returned to winning ways after his initial pro loss - an epic battle with Callum Smith in February - to earn a 20th pro success.
Various analysts thought the challenger, thirty-one, should have been awarded the decision but the British boxer endured the second loss of his 28-fight professional journey.
The Champion Secures Win in Uneventful Encounter
Buatsi's progress has stalled in recent times through management issues and long spells of inactivity.
Previously predicted as the next star of the British fight scene, he now is positioned trying to realize his potential - possibly in what seems like a last chance - under new promotional backing.
He started carefully, with the two boxers ready to overcommit in a cagey opening few rounds.
The challenger, whose single earlier setback came at the lower weight class against his former opponent in two years ago after suffering a hand injury during the bout, was competing in only his second fight at light-heavyweight.
By the halfway stage, Parker had kept it close, causing problems for the favorite, who operated with a tight guard to edge the cleaner periods.
The audience's tolerance diminished amid regular grabbing, with the challenger slipping to the canvas multiple occasions during clinches.
Parker closed well, but many rounds were hard to judge. Nevertheless, former world champion the boxing analyst - providing analysis for the broadcast network - had it strongly leaning for the challenger, giving Buatsi just only one stanza.
Buatsi moves on, but with the caliber of power puncher David Benavidez, and Russian champion Bivol ruling the division, the journey to titles - on this evidence - still seems difficult.
Hatton Homage Fuels Manchester Derby Fight
In the secondary fight, Arthur overcame Rea by close verdict in a excellent Manchester vs Manchester contest to capture the European light heavyweight championship.
The venue fell silent for a profoundly moving homage ahead of the contest as a bell ceremony tolled in remembrance of local hero Ricky Hatton, who passed away aged 46 in recently.
It was the initial significant broadcast match card in the location since the passing of Hatton. As the final chime ended, the stillness gave way to a chant of "there's only one Hatton".
The competition during the bout matched the sentiment. Arthur was floored by a left hook flush on the jaw in the second round, standing uncertainly to his feet on the count of eight.
But the traditional red attire of Arthur soon came forward again. Across the ring, Rea - in opposing blue - was damaged soon, heavy swelling appearing under his face as the fighters landed punishing blows.
The challenger, twenty-seven, landed power punches but the champion, thirty-four, fought fire with fire with effective uppercuts as the intensity seldom decreased over 12 rounds.
Arthur barely secured the decision with scoring tallies of draw, close margin and 115-112, but the two athletes were persistent and courageous, everything their idol Hatton embodied.
