British light heavyweight Buatsi made a winning yet disappointing beginning to his career under new promoters with a unanimous decision over Parker in Manchester.
The Olympic medal winner's punch output seemingly secured him a majority decision in a tight 10-round fight at the Manchester venue.
But Parker found success, connecting with powerful individual punches in a match where the techniques didn't complement each other, with constant grabbing spoiling the action.
Two judges had it 96-94 for to Buatsi, with the remaining official a 95-95.
"It wasn't my finest showing but what do champions do? They prevail," the victor said in the arena as he was met by disapproval from the spectators.
The Londoner bounced back after his first professional defeat - an thrilling contest with Callum Smith in February - to secure a 20th career victory.
Various analysts believed Parker, thirty-one, deserved the decision but the contender suffered the second defeat of his 28-match career.
Buatsi Claims Victory in Dull Match
The fighter's advancement has slowed in the past few years through promotional disputes and long spells of lack of fights.
Previously predicted as the next star of British boxing, he now stands trying to fulfil that early promise - possibly in what seems like a last chance - under fresh management.
He started carefully, with the two boxers willing to take risks in a cagey early stages.
The challenger, whose single earlier setback came at super-middleweight against John Ryder in two years ago after damaging his hand during the bout, was competing in only his second bout at the higher weight class.
By the halfway stage, the underdog had kept it close, creating difficulties for the favorite, who operated with a protective stance to barely take the more effective rounds.
The crowd's patience decreased amid regular grabbing, with the challenger slipping to the canvas multiple occasions during close exchanges.
Parker finished the stronger, but multiple periods were challenging to assess. Although, former world champion the boxing analyst - providing analysis for Dazn - had it significantly in favor for the challenger, giving Buatsi just only one stanza.
The victor progresses, but with the likes of heavy hitter David Benavidez, and Russian champion Bivol dominating the weight class, the road to world glory - on this evidence - still appears challenging.
Hatton Memorial Inspires Local Rivalry Battle
In the secondary fight, Lyndon Arthur defeated Bradley Rea by divided decision in a excellent all-Manchester battle to capture the European light heavyweight belt.
The venue fell silent for a profoundly moving tribute prior to the bout as ten bells rang in memory of the city's native Hatton, who passed away aged 46 in last month.
It was the first major televised fight night in the location since the passing of Hatton. As the last bell disappeared, the silence gave way to a roar of "there's just one Hatton".
The competition during the bout reflected the feeling. The eventual winner was knocked down by a left hook flush on the jaw in the initial stage, standing uncertainly to his footing on the count of eight.
But the Manchester United-red attire of Arthur soon advanced again. From the other corner, the opponent - in rival City blue - was marked up early, noticeable damage forming under his face as the pair landed punishing blows.
The challenger, 27, landed power punches but the champion, thirty-four, matched the intensity with effective uppercuts as the pace hardly diminished over twelve rounds.
The victor narrowly won the verdict with judges scorecards of draw, close margin and wider gap, but both fighters were relentless and courageous, everything their hero Hatton represented.
