British light heavyweight Buatsi earned a win yet disappointing start to life under recent management with a points victory over Parker in Manchester.
The Olympic bronze medallist's punch output appeared to earn him a majority decision in a competitive 10-round fight at the boxing arena.
But the challenger found success, delivering effective clean blows in a fight where the styles failed to mesh, with excessive holding disrupting the fight.
The officials scored it 96-94 for to Buatsi, with the remaining official a 95-95 draw.
"It wasn't my finest performance but how do champions respond? They prevail," the victor said in the ringside as he was greeted with boos from the crowd.
The Londoner got back to winning after his first professional defeat - an memorable fight with Callum Smith in last winter - to earn a 20th career victory.
Some ringside observers thought the challenger, thirty-one, merited the victory but the contender endured the second loss of his 28-bout career.
Buatsi Secures Win in Uneventful Affair
The fighter's advancement has stalled in recent times through management issues and considerable time of inactivity.
Earlier forecast as the rising talent of UK boxing, he now finds himself trying to fulfil that early promise - perhaps in what feels like a last chance - under fresh management.
He started carefully, with the two boxers willing to take risks in a tentative initial rounds.
The challenger, whose single earlier setback came at the lower weight class against John Ryder in 2022 after suffering a hand injury mid-fight, was competing in only his second fight at the higher weight class.
By the halfway stage, Parker had kept it close, causing problems for Buatsi, who operated with a protective stance to narrowly win the cleaner periods.
The audience's tolerance wore thin amid constant clinching, with the challenger falling to the floor various instances during grappling.
Parker ended stronger, but many rounds were hard to judge. However, ex-champion the boxing analyst - commentating on Dazn - had it strongly leaning for Parker, giving Buatsi just one round.
The victor progresses, but with the likes of hard hitter Benavidez, and Eastern European fighter Bivol leading the category, the road to world glory - on this evidence - still appears challenging.
Ricky Hatton Tribute Motivates Local Rivalry Contest
In the supporting bout, Lyndon Arthur overcame Rea by close verdict in a terrific all-Manchester battle to capture the European light heavyweight title.
The venue fell silent for a extremely emotional tribute prior to the bout as ten bells tolled in remembrance of the city's native Hatton, who passed away aged 46 in September.
It was the first major televised fight night in the urban center since Hatton's death. As the last bell faded, the stillness became a chant of "there's only one Hatton".
The competition inside the ropes matched the feeling. The eventual winner was knocked down by a powerful punch squarely on the face in the second round, standing uncertainly to his feet on the eight count.
But the Manchester United-red shorts of the fighter soon came forward again. On the opposite side, Rea - in rival City blue - was damaged soon, noticeable damage appearing under his orbital area as the fighters landed punishing blows.
Rea, 27, connected with strong blows but the champion, thirty-four, matched the intensity with powerful upward punches as the pace hardly diminished over twelve rounds.
Arthur narrowly won the result with official scores of even score, 115-113 and clearer margin, but the two athletes were persistent and courageous, everything their hero Hatton stood for.
