British light-heavyweight Buatsi made a winning yet unimpressive debut to his career under recent management with a decision win over Parker in the Manchester arena.
The Olympic medal winner's activity level likely gained him a split decision in a tight 10-round bout at the Co-op Live Arena.
But the challenger had his moments, landing some telling clean blows in a fight where the approaches didn't complement each other, with frequent clinching disrupting the fight.
The scoring judges had it 96-94 in favor to Buatsi, with the third a 95-95 draw.
"It wasn't my best showing but what do champions do? They win," the victor said in the ring as he was met by negative reactions from the audience.
The British fighter returned to winning ways after his maiden career setback - an memorable fight with Callum Smith in February - to secure a 20th career victory.
Some ringside observers thought the challenger, 31, deserved the win but the contender endured the second setback of his 28-bout career.
Buatsi Claims Triumph in Slow Affair
His career development has stalled in recent years through contract problems and long spells of inactivity.
Once tipped as the next star of the British fight scene, he now finds himself trying to realize his potential - possibly in what appears to be a last chance - under recent support.
He opened tentatively, with neither fighter willing to take risks in a calculated early stages.
The challenger, whose single earlier setback came at super middleweight against his former opponent in 2022 after damaging his hand in the contest, was participating in his second fight at light heavyweight.
By the middle rounds, the underdog had made it competitive, making life awkward for Buatsi, who operated with a protective stance to narrowly win the cleaner stanzas.
The spectators' acceptance wore thin because of frequent holding, with Parker going down several times during close exchanges.
Parker finished the stronger, but multiple periods were hard to judge. However, former world champion Carl Frampton - working with Dazn - had it significantly in favor for the challenger, awarding Buatsi just a single round.
The victor moves on, but with the likes of power puncher Benavidez, and Russian star Bivol ruling the division, the path to championship success - judging by this showing - still appears challenging.
Hatton Memorial Inspires Manchester Derby Battle
In the supporting bout, Arthur overcame Rea by split-decision in a outstanding Manchester vs Manchester battle to capture the European light-heavyweight title.
The venue became quiet for a profoundly moving tribute prior to the bout as a bell ceremony rang in remembrance of local hero Hatton, who died 46 years old in recently.
It was the first major televised match card in the city since Hatton's passing. As the last bell faded, the quiet gave way to a shout of "there's only one Hatton".
The competition within the ring equaled the sentiment. Arthur was sent to the canvas by a significant blow squarely on the jaw in the initial stage, standing uncertainly to his position on the eight count.
But the Manchester United-red equipment of Arthur soon advanced again. From the other corner, the opponent - in rival City blue - was marked up early, noticeable damage forming under his face as the duo traded punishing blows.
Rea, twenty-seven, delivered effective strikes but Arthur, 34, responded in kind with stinging uppercuts as the intensity hardly diminished over twelve rounds.
Arthur narrowly won the decision with judges scorecards of 114-114, 115-113 and wider gap, but each competitor were determined and brave, everything their hero the legend embodied.
