British light-heavyweight Buatsi earned a win yet disappointing debut to his career under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic medal winner's work rate seemingly secured him a judges' verdict in a close ten-round bout at the Co-op Live Arena.
But the challenger had his moments, landing some telling clean blows in a contest where the styles never quite clicked, with constant grabbing disrupting the action.
The scoring judges judged it 96-94 in favor to Buatsi, with the remaining official a 95-95 draw.
"It wasn't my greatest performance but what defines champions? They succeed," the victor stated in the ringside as he was faced negative reactions from the crowd.
The Londoner got back to winning after his first professional defeat - an epic battle with Callum Smith in last winter - to earn a twentieth professional win.
Various analysts believed Parker, 31, should have been awarded the decision but the contender suffered the second loss of his 28-bout professional journey.
The Champion Earns Win in Uneventful Encounter
His career development has stalled in the past few years through promotional disputes and long spells of inactivity.
Earlier forecast as the rising talent of UK boxing, he now is positioned trying to fulfil that early promise - maybe in what appears to be a ultimate opportunity - under fresh management.
He opened tentatively, with neither fighter ready to overcommit in a cagey opening few rounds.
The challenger, whose sole prior loss came at super middleweight against John Ryder in 2022 after breaking his hand in the contest, was making just his second fight at light-heavyweight.
By the halfway stage, the underdog had kept it close, making life awkward for Buatsi, who utilized a tight guard to edge the cleaner stanzas.
The audience's tolerance diminished during constant clinching, with Parker slipping to the canvas various instances during close exchanges.
Parker ended stronger, but multiple periods were hard to judge. However, previous titleholder the boxing analyst - commentating on Dazn - had it strongly leaning for Parker, scoring for Buatsi just only one stanza.
The victor moves on, but with the presence of power puncher David Benavidez, and Russian champion Bivol ruling the division, the journey to titles - judging by this showing - still appears challenging.
Ricky Hatton Memorial Fuels Manchester Derby Battle
In the secondary fight, Lyndon Arthur beat Bradley Rea by divided decision in a excellent local rivalry contest to win the European light-heavyweight championship.
The venue fell silent for a extremely emotional memorial ahead of the contest as ten bells tolled in memory of local hero Ricky Hatton, who succumbed at age 46 in recently.
It was the initial significant broadcast match card in the urban center since Hatton's death. As the concluding sound disappeared, the quiet became a shout of "there's only one Hatton".
The competition inside the ropes reflected the sentiment. Arthur was sent to the canvas by a significant blow flush on the chin in the second round, getting up unsteadily to his position on the referee's count.
But the red-colored equipment of Arthur soon came forward again. From the other corner, Rea - in opposing blue - was damaged quickly, significant bruising developing under his orbital area as the pair landed powerful shots.
Rea, twenty-seven, landed power punches but the champion, 34, fought fire with fire with stinging uppercuts as the pace hardly diminished over 12 rounds.
The victor narrowly won the result with official scores of 114-114, 115-113 and wider gap, but both fighters were relentless and courageous, all the qualities their idol Hatton represented.
