British light-heavyweight Buatsi secured a victory yet disappointing beginning to life under new promoters with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's work rate likely gained him a majority decision in a close 10-round contest at the Co-op Live Arena.
But Parker had his moments, connecting with powerful individual punches in a contest where the styles never quite clicked, with frequent clinching disrupting the action.
The officials judged it 96-94 in favor to the winner, with the remaining official a 95-95 draw.
"It wasn't my greatest showing but how do champions respond? They win," the victor said in the ringside as he was faced boos from the audience.
The British fighter got back to winning after his first professional defeat - an epic battle with his previous opponent in last winter - to secure a twentieth professional win.
Some ringside observers believed Parker, 31, merited the decision but the contender suffered the second loss of his 28-fight career.
Buatsi Earns Win in Uneventful Encounter
The fighter's advancement has plateaued in recent years through management issues and considerable time of inactivity.
Earlier forecast as the next star of the British fight scene, he now stands trying to fulfil that early promise - perhaps in what seems like a final roll of the dice - under recent support.
He started carefully, with the two boxers willing to overcommit 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 fight at light heavyweight.
By the middle rounds, Parker had stayed in contention, making life awkward for the favorite, who operated with a protective stance to narrowly win the more effective stanzas.
The spectators' acceptance diminished during regular grabbing, with the challenger falling to the floor multiple occasions during grappling.
The underdog ended stronger, but several stanzas were difficult to score. However, ex-champion Carl Frampton - commentating on the broadcast network - had it significantly in favor for the challenger, scoring for Buatsi just only one stanza.
The victor advances, but with the caliber of hard hitter David Benavidez, and Eastern European fighter Bivol dominating the weight class, the road to world glory - judging by this showing - still looks steep.
Hatton Homage Fuels Local Rivalry Contest
In the co-main event, Lyndon Arthur defeated Bradley Rea by split-decision in a outstanding local rivalry battle to win the continental belt.
The arena became quiet for a extremely emotional tribute before the fight as a bell ceremony tolled in remembrance of Manchester's own Hatton, who succumbed at age 46 in last month.
It was the first major televised match card in the city since the passing of Hatton. As the last bell disappeared, the stillness gave way to a shout of "only one Hatton".
The competition within the ring equaled the sentiment. The eventual winner was knocked down by a significant blow squarely on the face in the initial stage, rising shakily to his position on the eight count.
But the traditional red equipment of the fighter soon came forward again. Across the ring, Rea - in rival City blue - was marked up quickly, noticeable damage appearing under his eye as the fighters traded punishing blows.
The challenger, 27, landed power punches but the champion, thirty-four, fought fire with fire with powerful upward punches as the pace seldom decreased over 12 rounds.
Arthur narrowly won the decision with scoring tallies of even score, narrow lead and 115-112, but each competitor were relentless and brave, all the qualities their idol Hatton stood for.
