British light-heavyweight Buatsi made a winning yet underwhelming start to his career under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's activity level likely gained him a judges' verdict in a close 10-round bout at the Manchester venue.
But Parker had his moments, connecting with powerful clean blows in a contest where the techniques failed to mesh, with frequent clinching spoiling the action.
The scoring judges judged it 96-94 in favor to the winner, with the third a even score.
"It wasn't my finest display but what defines champions? They succeed," Buatsi commented in the ringside as he was greeted with boos from the spectators.
The British fighter got back to winning after his first professional defeat - an memorable fight with his previous opponent in last winter - to earn a 20th pro success.
Several boxing experts thought Parker, 31, should have been awarded the decision but the British boxer endured the second defeat of his 28-match professional journey.
Buatsi Secures Victory in Dull Affair
The fighter's advancement has slowed in recent years through promotional disputes and considerable time of lack of fights.
Previously predicted as the future of the British fight scene, he now stands trying to achieve expectations - perhaps in what appears to be a ultimate opportunity - under fresh management.
He began cautiously, with the two boxers willing to be aggressive in a cagey opening few rounds.
Parker, whose only previous defeat came at super-middleweight against John Ryder in 2022 after suffering a hand injury in the contest, was competing in only his second appearance at the higher weight class.
By the halfway stage, Parker had stayed in contention, making life awkward for Buatsi, who operated with a solid defense to narrowly win the more effective stanzas.
The crowd's patience wore thin during frequent holding, with Parker slipping to the canvas various instances during close exchanges.
Parker finished the stronger, but multiple periods were difficult to score. Although, previous titleholder Carl Frampton - providing analysis for the broadcast network - had it particularly wide for Parker, scoring for Buatsi just one round.
Buatsi moves on, but with the caliber of heavy hitter Benavidez, and Eastern European fighter Bivol ruling the division, the journey to titles - based on this performance - still seems difficult.
Hatton Tribute Motivates All-Manchester Fight
In the co-main event, Lyndon Arthur overcame Bradley Rea by split-decision in a outstanding all-Manchester battle to secure the European light heavyweight title.
The stadium grew hushed for a deeply poignant memorial ahead of the contest as ten bells tolled in remembrance of Manchester's own Hatton, who succumbed aged 46 in September.
It was the initial significant broadcast match card in the urban center since the passing of Hatton. As the concluding sound disappeared, the quiet transitioned to a roar of "there's only one Hatton".
The fighting inside the ropes equaled the sentiment. The eventual winner was floored by a powerful punch flush on the jaw in the initial stage, standing uncertainly to his feet on the count of eight.
But the traditional red attire of the fighter soon pressed ahead again. Across the ring, the opponent - in opposing blue - was marked up early, noticeable damage developing under his face as the duo landed hard punches.
Rea, 27, connected with strong blows but the champion, thirty-four, matched the intensity with powerful upward punches as the action seldom decreased over 12 rounds.
Arthur edged the decision with judges scorecards of draw, 115-113 and clearer margin, but each competitor were relentless and full of heart, exactly what their idol the legend represented.
