British light-heavyweight Buatsi secured a victory yet underwhelming beginning to life under recent management with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's activity level appeared to earn him a judges' verdict in a close 10-round contest at the boxing arena.
But Parker had his moments, landing some telling clean blows in a contest where the approaches never quite clicked, with excessive holding marring the action.
Two judges scored it 96-94 in favor to Buatsi, with the remaining official a 95-95.
"It wasn't my finest display but what do champions do? They prevail," Buatsi commented in the ringside as he was greeted with disapproval from the spectators.
The British fighter got back to winning after his initial pro loss - an thrilling contest with Callum Smith in last winter - to secure a 20th career victory.
Some ringside observers thought Parker, thirty-one, deserved the decision but the contender endured the second defeat of his 28-fight professional journey.
The Champion Earns Triumph in Uneventful Encounter
His career development has stalled in the past few years through contract problems and extended periods of inactivity.
Earlier forecast as the next star of British boxing, he now is positioned trying to fulfil that early promise - possibly in what feels like a last chance - under new promotional backing.
He opened tentatively, with both competitors ready to be aggressive in a cagey early stages.
Parker, whose single earlier setback came at the lower weight class against his former opponent in 2022 after breaking his hand mid-fight, was making just his second bout at light heavyweight.
By the middle rounds, the underdog had made it competitive, creating difficulties for Buatsi, who worked behind a solid defense to edge the cleaner periods.
The crowd's patience diminished amid frequent holding, with the challenger going down various instances during clinches.
The underdog finished the stronger, but many rounds were hard to judge. Nevertheless, ex-champion the boxing analyst - commentating on Dazn - had it strongly leaning for Parker, awarding Buatsi just one round.
The victor advances, but with the presence of power puncher David Benavidez, and Eastern European fighter Bivol leading the category, the path to championship success - based on this performance - still seems difficult.
Hatton Tribute Fuels All-Manchester Battle
In the co-main event, Arthur overcame Rea by close verdict in a terrific local rivalry fight to win the European light heavyweight championship.
The stadium became quiet for a profoundly moving tribute prior to the bout as ten bells tolled in remembrance of Manchester's own Hatton, who passed away aged 46 in last month.
It was the first major televised boxing event in the location since the passing of Hatton. As the last bell faded, the quiet transitioned to a roar of "only one Ricky Hatton".
The fighting during the bout equaled the emotion. Arthur was knocked down by a left hook flush on the chin in the initial stage, getting up unsteadily to his position on the referee's count.
But the Manchester United-red attire of Arthur soon pressed ahead again. Across the ring, Rea - in opposing blue - was damaged quickly, noticeable damage appearing under his eye as the fighters landed powerful shots.
Rea, twenty-seven, landed power punches but the champion, 34, matched the intensity with powerful upward punches as the pace hardly diminished over the full distance.
The victor narrowly won the verdict with judges scorecards of even score, narrow lead and 115-112, but each competitor were relentless and courageous, all the qualities their inspiration Hatton represented.
