UK light-heavyweight Joshua Buatsi made a winning yet unimpressive beginning to life under recent management with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's work rate likely gained him a split decision in a tight 10-round bout at the Co-op Live Arena.
But the challenger found success, landing some telling individual punches in a match where the approaches didn't complement each other, with excessive holding marring the action.
Two judges had it 96-94 for to Buatsi, with the third a even score.
"It wasn't my greatest showing but how do champions respond? They succeed," the victor said in the ringside as he was faced disapproval from the spectators.
The Londoner got back to winning after his first professional defeat - an memorable fight with his previous opponent in February - to secure a 20th career victory.
Various analysts believed Parker, thirty-one, merited the victory but the contender experienced the second loss of his 28-fight professional journey.
Buatsi Earns Victory in Slow Match
His career development has slowed in recent years through management issues and extended periods of lack of fights.
Previously predicted as the future of UK boxing, he now is positioned trying to achieve expectations - possibly in what feels like a last chance - under fresh management.
He started carefully, with both competitors ready to overcommit in a cagey opening few rounds.
The challenger, whose single earlier setback came at super-middleweight against his former opponent in two years ago after suffering a hand injury mid-fight, was making just his second fight at light heavyweight.
By the halfway stage, the underdog had stayed in contention, making life awkward for the favorite, who worked behind a tight guard to narrowly win the better stanzas.
The crowd's patience diminished during constant clinching, with the challenger slipping to the canvas various instances during close exchanges.
Parker ended stronger, but multiple periods were hard to judge. However, previous titleholder Carl Frampton - providing analysis for the broadcast network - had it strongly leaning for the challenger, giving Buatsi just one round.
The victor advances, but with the likes of power puncher David Benavidez, and Russian champion Dmitry Bivol leading the category, the journey to titles - based on this performance - still appears challenging.
Ricky Hatton Tribute Fuels Manchester Derby Contest
In the co-main event, Lyndon Arthur beat Rea by divided decision in a terrific all-Manchester contest to capture the European light-heavyweight championship.
The arena became quiet for a profoundly moving homage before the fight as a bell ceremony tolled in remembrance of Manchester's own Hatton, who passed away at age 46 in September.
It was the premier televised boxing event in the urban center since the passing of Hatton. As the final chime faded, the stillness transitioned to a roar of "there's just one Hatton".
The fighting within the ring reflected the feeling. The eventual winner was floored by a left hook directly to the chin in the early round, standing uncertainly to his feet on the eight count.
But the Manchester United-red attire of Arthur soon pressed ahead again. From the other corner, Rea - in competitive blue - was marked up soon, heavy swelling developing under his face as the pair landed powerful shots.
The challenger, 27, delivered effective strikes but the champion, 34, matched the intensity with powerful upward punches as the action hardly diminished over twelve rounds.
The victor barely secured the result with judges scorecards of even score, narrow lead and clearer margin, but both fighters were relentless and brave, all the qualities their hero the legend represented.
