British light-heavyweight Buatsi earned a win yet underwhelming start to his career under recent management with a points victory over Parker in the Manchester arena.
The Olympic medal winner's work rate likely gained him a majority decision in a tight ten-round bout at the boxing arena.
But the challenger found success, landing some telling single shots in a match where the techniques failed to mesh, with excessive holding disrupting the contest.
Two judges had it 96-94 to the winner, with the third a even score.
"It wasn't my best showing but how do champions respond? They win," Buatsi commented in the ringside as he was greeted with boos from the audience.
The British fighter returned to winning ways after his first professional defeat - an thrilling contest with Callum Smith in February - to claim a 20th career victory.
Several boxing experts believed Parker, 31, deserved the win but the contender experienced the second defeat of his 28-fight career.
The Champion Earns Victory in Slow Match
Buatsi's progress has slowed in recent years through management issues and considerable time of inactivity.
Previously predicted as the future of British boxing, he now finds himself trying to realize his potential - possibly in what feels like a ultimate opportunity - under new promotional backing.
He began cautiously, with both competitors prepared to take risks in a cagey opening few rounds.
The challenger, whose only previous defeat came at super-middleweight against John Ryder in 2022 after damaging his hand during the bout, was making just his second bout at the higher weight class.
By the halfway stage, Parker had stayed in contention, causing problems for Buatsi, who worked behind a protective stance to narrowly win the cleaner stanzas.
The audience's tolerance diminished amid regular grabbing, with the challenger slipping to the canvas various instances during clinches.
Parker ended stronger, but several stanzas were hard to judge. Nevertheless, former world champion Carl Frampton - working with Dazn - had it strongly leaning for Parker, giving Buatsi just only one stanza.
The victor progresses, but with the caliber of heavy hitter Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the journey to titles - based on this performance - still seems difficult.
Ricky Hatton Tribute Inspires Local Rivalry Contest
In the secondary fight, Lyndon Arthur defeated Bradley Rea by close verdict in a terrific all-Manchester battle to capture the European light-heavyweight title.
The stadium became quiet for a extremely emotional memorial prior to the bout as ten bells sounded in remembrance of local hero Ricky Hatton, who passed away 46 years old in September.
It was the premier televised boxing event in the urban center since the passing of Hatton. As the final chime disappeared, the stillness transitioned to a shout of "there's only one Hatton".
The action during the bout equaled the feeling. Arthur was sent to the canvas by a powerful punch flush on the jaw in the initial stage, rising shakily to his position on the referee's count.
But the red-colored shorts of Arthur soon advanced again. On the opposite side, Rea - in opposing blue - was injured soon, noticeable damage developing under his eye as the fighters traded hard punches.
The challenger, twenty-seven, connected with strong blows but the champion, thirty-four, responded in kind with powerful upward punches as the intensity seldom decreased over the full distance.
Arthur narrowly won the decision with scoring tallies of even score, 115-113 and 115-112, but each competitor were persistent and full of heart, exactly what their inspiration Hatton represented.
