UK light heavyweight Buatsi earned a win yet unimpressive start to life under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic bronze medallist's work rate appeared to earn him a judges' verdict in a tight 10-round fight at the Co-op Live Arena.
But Parker landed significant shots, landing some telling clean blows in a match where the techniques failed to mesh, with frequent clinching disrupting the fight.
Two judges had it 96-94 for to the winner, with the remaining official a 95-95.
"It wasn't my greatest performance but what defines champions? They win," the victor said in the arena as he was met by negative reactions from the crowd.
The Londoner bounced back after his initial pro loss - an thrilling contest with his previous opponent in February - to claim a 20th pro success.
Various analysts felt Parker, 31, should have been awarded the decision but the British boxer endured the second setback of his 28-bout professional journey.
Buatsi Secures Triumph in Slow Encounter
The fighter's advancement has slowed in the past few years through contract problems and long spells of inactivity.
Earlier forecast as the next star of UK boxing, he now stands trying to realize his potential - maybe in what appears to be a final roll of the dice - under recent support.
He began cautiously, with neither fighter 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 participating in his second bout at light heavyweight.
By the fight's midpoint, Parker had made it competitive, making life awkward for Buatsi, who utilized a protective stance to edge the better rounds.
The audience's tolerance decreased amid constant clinching, with Parker falling to the floor several times during grappling.
The underdog ended stronger, but several stanzas were challenging to assess. Nevertheless, former world champion Carl Frampton - working with the broadcast network - had it significantly in favor for the challenger, scoring for Buatsi just one round.
The victor moves on, but with the presence of power puncher David Benavidez, and Eastern European fighter Bivol ruling the division, the journey to titles - based on this performance - still seems difficult.
Ricky Hatton Homage Fuels Local Rivalry Contest
In the secondary fight, Lyndon Arthur beat Rea by divided decision in a outstanding local rivalry fight to win the continental championship.
The stadium fell silent for a deeply poignant homage before the fight as 10 bells rang in honor of local hero Hatton, who succumbed aged 46 in last month.
It was the first major televised fight night in the city since the passing of Hatton. As the concluding sound faded, the stillness transitioned to a roar of "only one Ricky Hatton".
The action inside the ropes matched the sentiment. The eventual winner was sent to the canvas by a significant blow flush on the jaw in the second round, standing uncertainly to his position on the referee's count.
But the Manchester United-red attire of Arthur soon advanced again. From the other corner, the opponent - in competitive blue - was injured soon, significant bruising forming under his orbital area as the fighters traded powerful shots.
The challenger, 27, connected with strong blows but the champion, thirty-four, responded in kind with effective uppercuts as the action seldom decreased over the full distance.
The victor narrowly won the verdict with judges scorecards of 114-114, 115-113 and 115-112, but both fighters were persistent and courageous, all the qualities their inspiration the legend represented.
