British light heavyweight Buatsi earned a win yet unimpressive beginning to his career under new promoters with a points victory over Parker in Manchester.
The Olympic bronze medallist's activity level likely gained him a majority decision in a tight ten-round fight at the Co-op Live Arena.
But Parker landed significant shots, delivering effective single shots in a match where the approaches never quite clicked, with excessive holding disrupting the contest.
Two judges judged it 96-94 for to the winner, with the third a even score.
"It wasn't my best showing but how do champions respond? They succeed," the victor commented in the ringside as he was greeted with disapproval from the audience.
The Londoner returned to winning ways after his first professional defeat - an thrilling contest with Callum Smith in February - to claim a 20th pro success.
Some ringside observers felt the challenger, thirty-one, deserved the victory but the contender experienced the second setback of his 28-fight career.
Buatsi Earns Triumph in Dull Encounter
His career development has stalled in the past few years through management issues and extended periods of inactivity.
Previously predicted as the next star of UK boxing, he now is positioned trying to achieve expectations - maybe in what feels like a last chance - under new promotional backing.
He opened tentatively, with neither fighter willing to take risks in a tentative initial rounds.
Parker, whose only previous defeat came at super middleweight against John Ryder in two years ago after suffering a hand injury mid-fight, was participating in his second bout at light-heavyweight.
By the halfway stage, the underdog had made it competitive, causing problems for the favorite, who utilized a tight guard to barely take the cleaner rounds.
The spectators' acceptance diminished because of constant clinching, with the challenger slipping to the canvas various instances during close exchanges.
Parker closed well, but many rounds were difficult to score. Nevertheless, former world champion the boxing analyst - commentating on Dazn - had it particularly wide for Parker, scoring for Buatsi just a single round.
Buatsi advances, but with the caliber of hard hitter David Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the journey to titles - judging by this showing - still appears challenging.
Ricky Hatton Homage Inspires Local Rivalry Fight
In the co-main event, Lyndon Arthur defeated Bradley Rea by divided decision in a terrific Manchester vs Manchester contest to secure the European light heavyweight championship.
The arena became quiet for a extremely emotional homage prior to the bout as a bell ceremony rang in honor of the city's native Hatton, who passed away aged 46 in recently.
It was the first major televised fight night in the urban center since the passing of Hatton. As the last bell disappeared, the quiet transitioned to a chant of "there's only one Hatton".
The action during the bout reflected the sentiment. The eventual winner was sent to the canvas by a left hook directly to the face in the second round, getting up unsteadily to his feet on the count of eight.
But the Manchester United-red attire of the fighter soon came forward again. On the opposite side, Rea - in competitive blue - was marked up quickly, noticeable damage forming under his face as the fighters landed powerful shots.
The challenger, twenty-seven, landed power punches but Arthur, thirty-four, responded in kind with stinging uppercuts as the intensity hardly diminished over the full distance.
Arthur barely secured the verdict with scoring tallies of even score, narrow lead and 115-112, but each competitor were determined and courageous, everything their hero the legend embodied.
