UK light heavyweight Buatsi earned a win yet disappointing start to life under recent management with a decision win over Parker in the Manchester arena.
The Olympic medal winner's punch output likely gained him a split decision in a tight ten-round fight at the Co-op Live Arena.
But Parker found success, delivering effective single shots in a fight where the styles failed to mesh, with excessive holding spoiling the action.
The scoring judges had it 96-94 in favor to the winner, with the remaining official a 95-95 draw.
"It wasn't my finest showing but what defines champions? They win," the victor commented in the ring as he was met by disapproval from the crowd.
The Londoner got back to winning after his maiden career setback - an epic battle with his previous opponent in last winter - to claim a 20th pro success.
Various analysts believed Parker, 31, should have been awarded the decision but the Derbyshire fighter experienced the second defeat of his 28-match professional journey.
The Champion Secures Win in Dull Affair
Buatsi's progress has stalled in recent years through contract problems and extended periods of lack of fights.
Once tipped as the next star of UK boxing, he now finds himself trying to realize his potential - perhaps in what appears to be a last chance - under new promotional backing.
He opened tentatively, with both competitors ready to be aggressive in a tentative initial rounds.
Parker, whose sole prior loss came at super-middleweight against his former opponent in 2022 after suffering a hand injury in the contest, was competing in only his second bout at the higher weight class.
By the middle rounds, Parker had kept it close, creating difficulties for Buatsi, who operated with a tight guard to narrowly win the cleaner stanzas.
The spectators' acceptance decreased during frequent holding, with Parker going down various instances during clinches.
Parker closed well, but several stanzas were difficult to score. However, previous titleholder the boxing analyst - working with Dazn - had it significantly in favor for Parker, scoring for Buatsi just one round.
The victor moves on, but with the caliber of heavy hitter David Benavidez, and Russian star Dmitry Bivol ruling the division, the path to championship success - based on this performance - still appears challenging.
Ricky Hatton Tribute Fuels Manchester Derby Fight
In the supporting bout, Lyndon Arthur defeated Rea by divided decision in a terrific all-Manchester battle to win the European light heavyweight championship.
The venue fell silent for a deeply poignant tribute ahead of the contest as 10 bells tolled in honor of local hero Ricky Hatton, who succumbed at age 46 in September.
It was the premier televised boxing event in the location since the passing of Hatton. As the concluding sound disappeared, the silence transitioned to a roar of "there's only one Ricky Hatton".
The action during the bout matched the feeling. Arthur was floored by a powerful punch directly to the face in the initial stage, standing uncertainly to his footing on the count of eight.
But the traditional red equipment of Arthur soon came forward again. On the opposite side, the opponent - in opposing blue - was injured quickly, noticeable damage developing under his face as the pair traded powerful shots.
Rea, 27, landed power punches but the champion, 34, fought fire with fire with stinging uppercuts as the action seldom decreased over the full distance.
Arthur edged the verdict with scoring tallies of 114-114, close margin and clearer margin, but both fighters were relentless and brave, all the qualities their idol Hatton stood for.
