UK light heavyweight Buatsi secured a victory yet underwhelming start to his career under new promoters with a decision win over Parker in Manchester.
The Olympic bronze medallist's punch output appeared to earn him a split decision in a tight ten-round bout at the boxing arena.
But Parker found success, delivering effective single shots in a fight where the approaches never quite clicked, with frequent clinching spoiling the action.
The scoring judges judged it 96-94 to Buatsi, with the remaining official a 95-95.
"It wasn't my greatest display but how do champions respond? They prevail," the victor said in the arena as he was met by negative reactions from the crowd.
The British fighter got back to winning after his initial pro loss - an memorable fight with Callum Smith in last winter - to claim a 20th career victory.
Various analysts felt the challenger, thirty-one, should have been awarded the victory but the contender suffered the second defeat of his 28-match professional journey.
Buatsi Earns Win in Uneventful Match
His career development has stalled in recent times through management issues and long spells of lack of fights.
Once tipped as the future of UK boxing, he now stands trying to realize his potential - maybe in what seems like a final roll of the dice - under recent support.
He began cautiously, with both competitors prepared to overcommit in a cagey early stages.
Parker, whose single earlier setback came at super middleweight against John Ryder in 2022 after suffering a hand injury mid-fight, was making just his second appearance at light heavyweight.
By the middle rounds, Parker had kept it close, making life awkward for the favorite, who operated with a protective stance to narrowly win the better stanzas.
The audience's tolerance wore thin amid regular grabbing, with Parker slipping to the canvas various instances during clinches.
Parker closed well, but multiple periods were challenging to assess. However, previous titleholder Carl Frampton - providing analysis for Dazn - had it strongly leaning for the challenger, giving Buatsi just only one stanza.
The victor progresses, but with the presence of hard hitter David Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the road to world glory - on this evidence - still looks steep.
Hatton Homage Inspires All-Manchester Battle
In the supporting bout, Arthur beat Rea by divided decision in a terrific Manchester vs Manchester contest to win the European light heavyweight title.
The venue grew hushed for a profoundly moving memorial ahead of the contest as 10 bells tolled in remembrance of Manchester's own Ricky Hatton, who passed away at age 46 in recently.
It was the initial significant broadcast boxing event in the city since Hatton's passing. As the final chime faded, the stillness gave way to a shout of "only one Hatton".
The fighting inside the ropes matched the sentiment. Arthur was floored by a significant blow squarely on the jaw in the early round, rising shakily to his position on the referee's count.
But the traditional red attire of the fighter soon advanced again. From the other corner, the opponent - in competitive blue - was marked up early, significant bruising forming under his eye as the pair exchanged punishing blows.
The challenger, 27, landed power punches but the champion, thirty-four, fought fire with fire with effective uppercuts as the pace seldom decreased over twelve rounds.
The victor narrowly won the verdict with scoring tallies of 114-114, narrow lead and clearer margin, but the two athletes were persistent and courageous, all the qualities their idol the legend represented.
