British light heavyweight Buatsi secured a victory yet unimpressive debut to his career under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's work rate appeared to earn him a majority decision in a close 10-round contest at the Co-op Live Arena.
But the challenger found success, landing some telling single shots in a match where the approaches didn't complement each other, with constant grabbing marring the fight.
The officials judged it 96-94 for to Buatsi, with the third a 95-95.
"It wasn't my finest performance but how do champions respond? They win," the victor stated in the ring as he was met by disapproval from the audience.
The Londoner got back to winning after his first professional defeat - an epic battle with Callum Smith in February - to earn a 20th career victory.
Various analysts believed Parker, 31, merited the win but the British boxer endured the second loss of his 28-bout career.
Buatsi Claims Triumph in Dull Affair
His career development has slowed in recent years through management issues and long spells of inactivity.
Once tipped as the future of British boxing, he now is positioned trying to achieve expectations - maybe in what feels like a last chance - under new promotional backing.
He started carefully, with neither fighter prepared to take risks in a calculated early stages.
Parker, whose only previous defeat came at super middleweight against his former opponent in two years ago after damaging his hand during the bout, was participating in his second appearance at light-heavyweight.
By the middle rounds, the underdog had kept it close, creating difficulties for the favorite, who worked behind a protective stance to narrowly win the more effective stanzas.
The audience's tolerance diminished amid regular grabbing, with the challenger falling to the floor several times during close exchanges.
Parker finished the stronger, but multiple periods were difficult to score. However, ex-champion Carl Frampton - providing analysis for the broadcast network - had it strongly leaning for Parker, scoring for Buatsi just a single round.
The victor progresses, but with the likes of heavy hitter David Benavidez, and Russian champion Bivol ruling the division, the path to championship success - on this evidence - still seems difficult.
Hatton Homage Motivates Local Rivalry Battle
In the co-main event, Arthur defeated Bradley Rea by close verdict in a outstanding Manchester vs Manchester contest to secure the European light heavyweight championship.
The arena fell silent for a extremely emotional homage prior to the bout as a bell ceremony rang in remembrance of Manchester's own Ricky Hatton, who succumbed 46 years old in recently.
It was the first major televised match card in the city since Hatton's passing. As the last bell ended, the quiet became a shout of "there's only one Ricky Hatton".
The action inside the ropes reflected the sentiment. Arthur was knocked down by a left hook directly to the chin in the initial stage, rising shakily to his feet on the count of eight.
But the traditional red equipment of the fighter soon pressed ahead again. On the opposite side, the opponent - in rival City blue - was marked up quickly, noticeable damage appearing under his orbital area as the pair landed hard punches.
Rea, twenty-seven, landed power punches but the champion, thirty-four, matched the intensity with powerful upward punches as the pace hardly diminished over the full distance.
Arthur narrowly won the decision with judges scorecards of 114-114, close margin and wider gap, but each competitor were persistent and full of heart, exactly what their idol Hatton embodied.
