UK light-heavyweight Buatsi earned a win yet unimpressive debut to his career under new promoters with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output appeared to earn him a judges' verdict in a competitive 10-round bout at the Manchester venue.
But Parker landed significant shots, landing some telling individual punches in a match where the techniques failed to mesh, with excessive holding spoiling the action.
The officials judged it 96-94 for to Buatsi, with the remaining official a 95-95.
"It wasn't my finest performance but how do champions respond? They succeed," Buatsi stated in the ringside as he was greeted with disapproval from the audience.
The British fighter returned to winning ways after his first professional defeat - an epic battle with Callum Smith in last winter - to earn a 20th pro success.
Various analysts felt Parker, thirty-one, deserved the win but the contender endured the second loss of his 28-match professional journey.
The Champion Secures Triumph in Dull Match
The fighter's advancement has stalled in recent times through contract problems and considerable time of inactivity.
Previously predicted as the next star of UK boxing, he now finds himself trying to realize his potential - maybe in what seems like a ultimate opportunity - under new promotional backing.
He started carefully, with the two boxers ready to be aggressive in a cagey opening few rounds.
The challenger, whose only previous defeat came at super middleweight against his former opponent in 2022 after damaging his hand during the bout, was making just his second appearance at light heavyweight.
By the halfway stage, Parker had made it competitive, making life awkward for the favorite, who utilized a tight guard to edge the cleaner rounds.
The audience's tolerance diminished amid constant clinching, with the challenger slipping to the canvas various instances during clinches.
The underdog ended stronger, but many rounds were difficult to score. However, former world champion Carl Frampton - commentating on the broadcast network - had it strongly leaning for Parker, scoring for Buatsi just one round.
Buatsi moves on, but with the likes of heavy hitter Benavidez, and Russian star Dmitry Bivol dominating the weight class, the journey to titles - judging by this showing - still seems difficult.
Ricky Hatton Memorial Fuels All-Manchester Contest
In the co-main event, Lyndon Arthur overcame Bradley Rea by split-decision in a terrific Manchester vs Manchester contest to secure the European light-heavyweight belt.
The stadium fell silent for a profoundly moving tribute before the fight as a bell ceremony tolled in memory of Manchester's own Ricky Hatton, who succumbed at age 46 in September.
It was the premier televised fight night in the urban center since Hatton's death. As the last bell ended, the quiet transitioned to a chant of "there's just one Ricky Hatton".
The fighting within the ring equaled the sentiment. The eventual winner was floored by a powerful punch squarely on the face in the second round, getting up unsteadily to his position on the referee's count.
But the traditional red equipment of the fighter soon came forward again. On the opposite side, the opponent - in opposing blue - was damaged quickly, noticeable damage forming under his orbital area as the duo landed powerful shots.
The challenger, twenty-seven, landed power punches but Arthur, 34, matched the intensity with powerful upward punches as the action rarely dropped over the full distance.
Arthur barely secured the decision with official scores of even score, close margin and clearer margin, but each competitor were persistent and brave, exactly what their hero Hatton stood for.
