British light heavyweight Buatsi made a winning yet disappointing start to life under recent management with a points victory over Parker in the Manchester arena.
The Olympic medal winner's work rate appeared to earn him a majority decision in a tight ten-round bout at the Co-op Live Arena.
But Parker found success, connecting with powerful individual punches in a match where the styles never quite clicked, with frequent clinching disrupting the fight.
The officials judged it 96-94 to Buatsi, with the third a even score.
"It wasn't my best performance but what do champions do? They win," the victor commented in the ring as he was greeted with disapproval from the crowd.
The Londoner bounced back after his maiden career setback - an memorable fight with his previous opponent in February - to earn a 20th pro success.
Several boxing experts thought Parker, 31, deserved the win but the British boxer endured the second setback of his 28-match professional journey.
The Champion Earns Victory in Uneventful Encounter
The fighter's advancement has plateaued in recent years through management issues and long spells of inactivity.
Previously predicted as the next star of the British fight scene, he now stands trying to fulfil that early promise - maybe in what appears to be a final roll of the dice - under new promotional backing.
He began cautiously, with both competitors ready to overcommit in a cagey early stages.
The challenger, whose only previous defeat came at the lower weight class against John Ryder in two years ago after damaging his hand during the bout, was participating in his second appearance at the higher weight class.
By the fight's midpoint, the underdog had made it competitive, creating difficulties for Buatsi, who utilized a solid defense to barely take the cleaner rounds.
The spectators' acceptance wore thin because of frequent holding, with the challenger slipping to the canvas various instances during grappling.
The underdog finished the stronger, but many rounds were hard to judge. However, ex-champion the boxing analyst - commentating on Dazn - had it significantly in favor for the challenger, scoring for Buatsi just a single round.
Buatsi moves on, but with the likes of hard hitter Benavidez, and Eastern European fighter Bivol leading the category, the path to championship success - judging by this showing - still seems difficult.
Hatton Memorial Motivates Manchester Derby Contest
In the secondary fight, Lyndon Arthur beat Bradley Rea by divided decision in a outstanding Manchester vs Manchester fight to win the European light-heavyweight championship.
The stadium fell silent for a extremely emotional homage ahead of the contest as ten bells sounded in honor of the city's native Ricky Hatton, who passed away at age 46 in September.
It was the premier televised fight night in the location since Hatton's death. As the last bell faded, the silence gave way to a shout of "there's just one Hatton".
The fighting within the ring reflected the feeling. The eventual winner was floored by a powerful punch squarely on the chin in the early round, standing uncertainly to his footing on the referee's count.
But the red-colored shorts of the fighter soon advanced again. From the other corner, the opponent - in rival City blue - was marked up soon, significant bruising appearing under his eye as the duo traded punishing blows.
Rea, twenty-seven, connected with strong blows but Arthur, 34, matched the intensity with powerful upward punches as the pace rarely dropped over 12 rounds.
Arthur edged the verdict with judges scorecards of even score, close margin and wider gap, but both fighters were persistent and courageous, everything their idol Hatton represented.
