UK light-heavyweight Buatsi secured a victory yet unimpressive debut to life under recent management with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output appeared to earn him a majority decision in a close 10-round bout at the Co-op Live Arena.
But Parker landed significant shots, delivering effective clean blows in a match where the techniques never quite clicked, with frequent clinching disrupting the action.
The scoring judges had it 96-94 to the winner, with the remaining official a 95-95 draw.
"It wasn't my greatest performance but what do champions do? They succeed," the victor said in the ring as he was met by disapproval from the audience.
The British fighter bounced back after his initial pro loss - an epic battle with his previous opponent in February - to secure a twentieth professional win.
Various analysts thought Parker, thirty-one, deserved the win but the British boxer experienced the second loss of his 28-fight career.
The Champion Earns Victory in Dull Encounter
Buatsi's progress has plateaued in recent times through contract problems and considerable time of inactivity.
Earlier forecast as the next star of the British fight scene, he now stands trying to achieve expectations - maybe in what appears to be a ultimate opportunity - under fresh management.
He started carefully, with the two boxers willing to overcommit in a cagey initial rounds.
Parker, whose sole prior loss came at the lower weight class against John Ryder in two years ago after breaking his hand mid-fight, was making just his second fight at light-heavyweight.
By the halfway stage, the underdog had made it competitive, causing problems for the favorite, who operated with a tight guard to barely take the more effective periods.
The spectators' acceptance diminished amid constant clinching, with Parker falling to the floor several times during grappling.
The underdog finished the stronger, but many rounds were hard to judge. Although, former world champion the boxing analyst - working with Dazn - had it significantly in favor for the challenger, awarding Buatsi just only one stanza.
The victor moves on, but with the presence of heavy hitter David Benavidez, and Eastern European fighter Bivol leading the category, the journey to titles - based on this performance - still seems difficult.
Hatton Memorial Motivates Local Rivalry Contest
In the secondary fight, Lyndon Arthur beat Rea by split-decision in a outstanding all-Manchester battle to secure the continental belt.
The stadium grew hushed for a deeply poignant memorial ahead of the contest as a bell ceremony rang in remembrance of the city's native Hatton, who succumbed at age 46 in September.
It was the first major televised fight night in the city since Hatton's passing. As the last bell faded, the silence became a chant of "there's only one Ricky Hatton".
The competition inside the ropes reflected the emotion. Arthur was floored by a left hook squarely on the chin in the second round, standing uncertainly to his position on the eight count.
But the red-colored attire of the fighter soon advanced again. On the opposite side, the opponent - in opposing blue - was marked up quickly, noticeable damage forming under his eye as the fighters traded punishing blows.
Rea, 27, connected with strong blows but the champion, thirty-four, matched the intensity with stinging uppercuts as the action hardly diminished over twelve rounds.
Arthur barely secured the result with scoring tallies of even score, 115-113 and 115-112, but both fighters were determined and courageous, all the qualities their inspiration the legend represented.
