UK light-heavyweight Buatsi made a winning yet disappointing debut to his career under new promoters with a points victory over Parker in Manchester.
The Olympic medal winner's activity level likely gained him a split decision in a close ten-round bout at the boxing arena.
But Parker found success, landing some telling single shots in a match where the approaches never quite clicked, with constant grabbing marring the contest.
The scoring judges scored it 96-94 in favor to Buatsi, with the remaining official a 95-95.
"It wasn't my finest display but how do champions respond? They succeed," the victor commented in the ring as he was met by disapproval from the crowd.
The British fighter bounced back after his maiden career setback - an epic battle with Callum Smith in last winter - to secure a twentieth professional win.
Various analysts believed the challenger, 31, merited the victory but the contender suffered the second loss of his 28-match career.
The Champion Claims Triumph in Uneventful Affair
His career development has stalled in recent times through promotional disputes and long spells of inactivity.
Earlier forecast as the future of UK boxing, he now stands trying to realize his potential - possibly in what seems like a final roll of the dice - under recent support.
He began cautiously, with both competitors ready to be aggressive in a tentative early stages.
Parker, whose sole prior loss came at super-middleweight against his former opponent in two years ago after suffering a hand injury during the bout, was participating in his second appearance at light heavyweight.
By the halfway stage, Parker had made it competitive, creating difficulties for the favorite, who operated with a tight guard to narrowly win the better rounds.
The spectators' acceptance decreased during frequent holding, with Parker going down several times during clinches.
Parker ended stronger, but multiple periods were challenging to assess. However, previous titleholder the boxing analyst - working with the broadcast network - had it strongly leaning for Parker, awarding Buatsi just only one stanza.
The victor moves on, but with the presence of heavy hitter David Benavidez, and Eastern European fighter Bivol dominating the weight class, the journey to titles - judging by this showing - still appears challenging.
Ricky Hatton Memorial Inspires Local Rivalry Fight
In the co-main event, Arthur defeated Rea by close verdict in a outstanding Manchester vs Manchester fight to capture the continental championship.
The venue fell silent for a extremely emotional homage ahead of the contest as ten bells tolled in honor of the city's native Hatton, who died aged 46 in recently.
It was the premier televised boxing event in the location since the passing of Hatton. As the last bell faded, the stillness transitioned to a roar of "there's just one Hatton".
The competition within the ring reflected the emotion. The eventual winner was floored by a significant blow directly to the face in the early round, rising shakily to his feet on the count of eight.
But the traditional red equipment of the fighter soon pressed ahead again. From the other corner, the opponent - in competitive blue - was marked up early, significant bruising developing under his eye as the fighters traded hard punches.
The challenger, 27, delivered effective strikes but the champion, 34, matched the intensity with effective uppercuts as the intensity hardly diminished over 12 rounds.
Arthur barely secured the verdict with scoring tallies of draw, narrow lead and clearer margin, but both fighters were persistent and brave, everything their inspiration the legend embodied.
