British light-heavyweight Buatsi earned a win yet disappointing start to his career under recent management with a points victory over Parker in Manchester.
The Olympic bronze medallist's activity level appeared to earn him a split decision in a competitive 10-round fight at the Manchester venue.
But Parker found success, delivering effective clean blows in a contest where the techniques failed to mesh, with frequent clinching spoiling the contest.
The officials judged it 96-94 to Buatsi, with the third a 95-95 draw.
"It wasn't my greatest showing but how do champions respond? They succeed," the victor stated in the ring as he was met by boos from the audience.
The British fighter bounced back after his first professional defeat - an epic battle with his previous opponent in last winter - to claim a 20th pro success.
Various analysts felt the challenger, 31, should have been awarded the decision but the British boxer experienced the second loss of his 28-bout professional journey.
Buatsi Claims Victory in Dull Encounter
His career development has stalled in recent times through promotional disputes and considerable time of lack of fights.
Previously predicted as the future of UK boxing, he now stands trying to fulfil that early promise - maybe in what seems like a last chance - under new promotional backing.
He opened tentatively, with neither fighter prepared to be aggressive in a cagey initial rounds.
Parker, whose sole prior loss came at super middleweight against John Ryder in 2022 after damaging his hand during the bout, was participating in his second appearance at the higher weight class.
By the middle rounds, the underdog had stayed in contention, making life awkward for Buatsi, who utilized a solid defense to edge the more effective stanzas.
The spectators' acceptance wore thin amid constant clinching, with Parker falling to the floor several times during grappling.
Parker ended stronger, but several stanzas were difficult to score. Nevertheless, former world champion the boxing analyst - commentating on Dazn - had it significantly in favor for Parker, awarding Buatsi just only one stanza.
Buatsi moves on, but with the caliber of heavy hitter Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the journey to titles - judging by this showing - still looks steep.
Hatton Tribute Inspires Manchester Derby Battle
In the secondary fight, Lyndon Arthur defeated Bradley Rea by divided decision in a terrific local rivalry contest to win the European light-heavyweight championship.
The stadium fell silent for a profoundly moving homage ahead of the contest as a bell ceremony tolled in remembrance of Manchester's own Ricky Hatton, who passed away at age 46 in last month.
It was the premier televised fight night in the location since Hatton's passing. As the last bell disappeared, the quiet transitioned to a shout of "only one Hatton".
The action during the bout matched the sentiment. Arthur was floored by a significant blow directly to the jaw in the second round, getting up unsteadily to his feet on the count of eight.
But the traditional red equipment of Arthur soon advanced again. Across the ring, the opponent - in rival City blue - was damaged soon, heavy swelling developing under his eye as the duo traded powerful shots.
The challenger, twenty-seven, delivered effective strikes but the champion, thirty-four, matched the intensity with powerful upward punches as the pace rarely dropped over the full distance.
Arthur narrowly won the verdict with official scores of 114-114, close margin and wider gap, but the two athletes were relentless and brave, all the qualities their idol the legend embodied.
