UK light-heavyweight Joshua Buatsi earned a win yet unimpressive beginning to his career under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's work rate appeared to earn him a majority decision in a tight 10-round fight at the boxing arena.
But Parker had his moments, delivering effective individual punches in a contest where the approaches didn't complement each other, with frequent clinching disrupting the contest.
Two judges had it 96-94 in favor to the winner, with the third a 95-95 draw.
"It wasn't my finest performance but what do champions do? They win," Buatsi stated in the ring as he was greeted with disapproval from the crowd.
The Londoner got back to winning after his maiden career setback - an epic battle with Callum Smith in February - to secure a 20th pro success.
Some ringside observers believed Parker, thirty-one, deserved the victory but the British boxer endured the second defeat of his 28-match career.
The Champion Claims Triumph in Slow Encounter
Buatsi's progress has slowed in recent times through contract problems and long spells of lack of fights.
Previously predicted as the rising talent of British boxing, he now stands trying to realize his potential - perhaps in what feels like a last chance - under recent support.
He began cautiously, with both competitors prepared to be aggressive in a calculated initial rounds.
The challenger, whose only previous defeat came at super-middleweight against John Ryder in two years ago after damaging his hand during the bout, was competing in only his second fight at light-heavyweight.
By the fight's midpoint, the underdog had kept it close, creating difficulties for the favorite, who worked behind a protective stance to narrowly win the cleaner periods.
The audience's tolerance decreased because of constant clinching, with Parker falling to the floor various instances during close exchanges.
Parker finished the stronger, but many rounds were challenging to assess. Nevertheless, ex-champion the boxing analyst - commentating on the broadcast network - had it particularly wide for Parker, giving Buatsi just only one stanza.
Buatsi progresses, but with the presence of power puncher Benavidez, and Russian star Bivol leading the category, the path to championship success - judging by this showing - still seems difficult.
Ricky Hatton Homage Inspires Local Rivalry Battle
In the supporting bout, Lyndon Arthur beat Rea by divided decision in a outstanding Manchester vs Manchester fight to secure the European light heavyweight championship.
The stadium fell silent for a profoundly moving homage prior to the bout as ten bells rang in honor of Manchester's own Hatton, who passed away 46 years old in September.
It was the premier televised match card in the location since the passing of Hatton. As the final chime ended, the silence gave way to a chant of "there's just one Ricky Hatton".
The action inside the ropes matched the sentiment. Arthur was floored by a powerful punch flush on the chin in the initial stage, rising shakily to his position on the eight count.
But the red-colored shorts of the fighter soon came forward again. Across the ring, Rea - in competitive blue - was damaged quickly, noticeable damage developing under his face as the fighters exchanged powerful shots.
Rea, 27, connected with strong blows but the champion, thirty-four, fought fire with fire with powerful upward punches as the intensity rarely dropped over twelve rounds.
Arthur narrowly won the result with official scores of draw, close margin and clearer margin, but each competitor were determined and full of heart, all the qualities their inspiration Hatton represented.
