British light-heavyweight Joshua Buatsi secured a victory yet underwhelming start to his career under recent management with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's work rate likely gained him a split decision in a close 10-round contest at the boxing arena.
But Parker landed significant shots, connecting with powerful clean blows in a match where the styles failed to mesh, with constant grabbing spoiling the contest.
Two judges had it 96-94 to the winner, with the remaining official a 95-95 draw.
"It wasn't my finest display but how do champions respond? They win," the victor commented in the arena as he was greeted with disapproval from the crowd.
The Londoner returned to winning ways after his first professional defeat - an thrilling contest with his previous opponent in last winter - to claim a 20th career victory.
Some ringside observers thought the challenger, 31, merited the victory but the contender endured the second loss of his 28-match career.
Buatsi Earns Win in Dull Affair
His career development has slowed in recent years through management issues and extended periods of lack of fights.
Once tipped as the rising talent of British boxing, he now is positioned trying to fulfil that early promise - perhaps in what seems like a last chance - under new promotional backing.
He opened tentatively, with neither fighter prepared to be aggressive in a cagey early stages.
Parker, whose only previous defeat came at super middleweight against his former opponent in two years ago after breaking his hand mid-fight, was participating in his second fight at the higher weight class.
By the halfway stage, Parker had kept it close, making life awkward for Buatsi, who worked behind a tight guard to narrowly win the more effective rounds.
The crowd's patience decreased because of constant clinching, with Parker going down several times during clinches.
The underdog finished the stronger, but multiple periods were challenging to assess. Although, previous titleholder the boxing analyst - commentating on Dazn - had it strongly leaning for Parker, scoring for Buatsi just a single round.
The victor progresses, but with the likes of heavy hitter David Benavidez, and Russian star Bivol leading the category, the path to championship success - judging by this showing - still appears challenging.
Hatton Memorial Fuels Manchester Derby Fight
In the supporting bout, Lyndon Arthur overcame Rea by close verdict in a terrific local rivalry contest to win the European light heavyweight title.
The venue fell silent for a deeply poignant homage ahead of the contest as ten bells tolled in memory of local hero Hatton, who succumbed at age 46 in recently.
It was the initial significant broadcast boxing event in the urban center since the passing of Hatton. As the final chime disappeared, the stillness gave way to a chant of "only one Ricky Hatton".
The competition inside the ropes reflected the feeling. Arthur was floored by a left hook directly to the jaw in the early round, standing uncertainly to his footing on the eight count.
But the red-colored equipment of the fighter soon pressed ahead again. From the other corner, the opponent - in competitive blue - was marked up quickly, heavy swelling developing under his face as the pair traded hard punches.
Rea, 27, delivered effective strikes but the champion, thirty-four, matched the intensity with powerful upward punches as the intensity rarely dropped over 12 rounds.
The victor edged the verdict with official scores of 114-114, 115-113 and clearer margin, but each competitor were determined and courageous, exactly what their idol the legend stood for.
