UK light heavyweight Buatsi secured a victory yet unimpressive start to life under recent management with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's activity level likely gained him a judges' verdict in a tight 10-round bout at the Manchester venue.
But the challenger had his moments, landing some telling individual punches in a contest where the techniques failed to mesh, with constant grabbing marring the contest.
Two judges judged it 96-94 to the winner, with the remaining official a even score.
"It wasn't my best display but what do champions do? They win," Buatsi commented in the ringside as he was met by disapproval from the crowd.
The Londoner bounced back after his initial pro loss - an epic battle with his previous opponent in February - to claim a 20th pro success.
Various analysts believed Parker, thirty-one, merited the decision but the contender suffered the second loss of his 28-bout professional journey.
The Champion Claims Triumph in Dull Match
Buatsi's progress has stalled in the past few years through promotional disputes and considerable time of lack of fights.
Once tipped as the future of British boxing, he now finds himself trying to achieve expectations - maybe in what feels like a last chance - under fresh management.
He began cautiously, with neither fighter willing to be aggressive in a calculated early stages.
Parker, whose sole prior loss came at super middleweight against John Ryder in two years ago after breaking his hand during the bout, was competing in only his second appearance at the higher weight class.
By the fight's midpoint, Parker had stayed in contention, making life awkward for the favorite, who worked behind a protective stance to barely take the more effective stanzas.
The spectators' acceptance wore thin amid constant clinching, with Parker falling to the floor multiple occasions during close exchanges.
The underdog ended stronger, but multiple periods were challenging to assess. However, ex-champion Carl Frampton - commentating on Dazn - had it particularly wide for Parker, giving Buatsi just a single round.
The victor moves on, but with the likes of power puncher David Benavidez, and Russian star Dmitry Bivol dominating the weight class, the road to world glory - judging by this showing - still seems difficult.
Ricky Hatton Tribute Motivates All-Manchester Battle
In the co-main event, Lyndon Arthur beat Rea by split-decision in a excellent Manchester vs Manchester contest to capture the continental championship.
The venue grew hushed for a profoundly moving tribute ahead of the contest as ten bells sounded in honor of Manchester's own Hatton, who passed away 46 years old in recently.
It was the initial significant broadcast match card in the urban center since Hatton's passing. As the concluding sound disappeared, the silence transitioned to a roar of "only one Ricky Hatton".
The action inside the ropes reflected the emotion. The eventual winner was floored by a powerful punch directly to the face in the initial stage, standing uncertainly to his feet on the count of eight.
But the Manchester United-red attire of the fighter soon pressed ahead again. From the other corner, the opponent - in rival City blue - was marked up early, noticeable damage forming under his eye as the pair exchanged punishing blows.
Rea, twenty-seven, landed power punches but Arthur, 34, fought fire with fire with stinging uppercuts as the pace seldom decreased over 12 rounds.
The victor narrowly won the decision with scoring tallies of 114-114, 115-113 and clearer margin, but the two athletes were determined and courageous, all the qualities their hero Hatton stood for.
