British light-heavyweight Buatsi made a winning yet unimpressive start to life under recent management with a decision win over Parker in Manchester.
The Olympic medal winner's activity level appeared to earn him a split decision in a competitive ten-round fight at the boxing arena.
But the challenger found success, landing some telling single shots in a fight where the approaches failed to mesh, with constant grabbing spoiling the action.
Two judges judged it 96-94 for to Buatsi, with the remaining official a 95-95.
"It wasn't my best display but what do champions do? They win," the victor commented in the arena as he was greeted with boos from the audience.
The Londoner bounced back after his initial pro loss - an epic battle with his previous opponent in February - to claim a 20th pro success.
Some ringside observers thought the challenger, thirty-one, deserved the decision but the contender endured the second setback of his 28-match career.
The Champion Claims Victory in Slow Match
The fighter's advancement has slowed in the past few years through contract problems and long spells of lack of fights.
Once tipped as the rising talent of the British fight scene, he now stands trying to realize his potential - possibly in what seems like a ultimate opportunity - under fresh management.
He began cautiously, with the two boxers willing to take risks in a tentative initial rounds.
The challenger, whose single earlier setback came at super middleweight against John Ryder in 2022 after suffering a hand injury mid-fight, was making just his second fight at light heavyweight.
By the fight's midpoint, the underdog had kept it close, making life awkward for the favorite, who operated with a tight guard to narrowly win the better periods.
The spectators' acceptance diminished amid constant clinching, with Parker going down multiple occasions during close exchanges.
Parker closed well, but multiple periods were hard to judge. Nevertheless, previous titleholder Carl Frampton - working with Dazn - had it particularly wide for the challenger, giving Buatsi just only one stanza.
The victor progresses, but with the caliber of power puncher Benavidez, and Russian champion Bivol ruling the division, the journey to titles - based on this performance - still appears challenging.
Hatton Homage Fuels Manchester Derby Fight
In the co-main event, Lyndon Arthur defeated Bradley Rea by close verdict in a excellent all-Manchester contest to secure the European light heavyweight championship.
The stadium grew hushed for a profoundly moving tribute ahead of the contest as 10 bells tolled in memory of the city's native Ricky Hatton, who succumbed 46 years old in recently.
It was the first major televised match card in the location since the passing of Hatton. As the last bell ended, the silence gave way to a shout of "only one Hatton".
The fighting within the ring reflected the sentiment. Arthur was floored by a powerful punch flush on the jaw in the early round, standing uncertainly to his feet on the count of eight.
But the traditional red equipment of Arthur soon advanced again. On the opposite side, Rea - in competitive blue - was marked up soon, heavy swelling appearing under his eye as the pair traded powerful shots.
Rea, 27, connected with strong blows but Arthur, 34, matched the intensity with effective uppercuts as the action rarely dropped over twelve rounds.
The victor barely secured the decision with judges scorecards of 114-114, 115-113 and 115-112, but each competitor were relentless and courageous, everything their hero the legend represented.
