UK light-heavyweight Buatsi made a winning yet unimpressive start to his career under recent management with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output likely gained him a judges' verdict in a competitive 10-round bout at the boxing arena.
But Parker had his moments, delivering effective clean blows in a match where the techniques never quite clicked, with excessive holding spoiling the fight.
The officials scored it 96-94 to the winner, with the remaining official a 95-95 draw.
"It wasn't my finest showing but how do champions respond? They win," the victor stated in the arena as he was greeted with negative reactions from the audience.
The British fighter returned to winning ways after his initial pro loss - an thrilling contest with Callum Smith in February - to claim a 20th pro success.
Several boxing experts thought the challenger, 31, merited the victory but the Derbyshire fighter suffered the second setback of his 28-fight career.
Buatsi Secures Victory in Dull Encounter
The fighter's advancement has stalled in recent times through promotional disputes and extended periods of inactivity.
Once tipped as the rising talent of UK boxing, he now stands trying to realize his potential - maybe in what appears to be a last chance - under new promotional backing.
He began cautiously, with both competitors ready to be aggressive in a calculated initial rounds.
Parker, whose sole prior loss came at super-middleweight against John Ryder in 2022 after damaging his hand in the contest, was making just his second fight at the higher weight class.
By the middle rounds, Parker had made it competitive, making life awkward for Buatsi, who utilized a solid defense to edge the better rounds.
The spectators' acceptance wore thin during constant clinching, with Parker falling to the floor multiple occasions during grappling.
Parker closed well, but several stanzas were difficult to score. However, previous titleholder Carl Frampton - working with the broadcast network - had it significantly in favor for the challenger, awarding Buatsi just a single round.
Buatsi progresses, but with the likes of heavy hitter David Benavidez, and Russian star Bivol dominating the weight class, the path to championship success - on this evidence - still seems difficult.
Ricky Hatton Tribute Motivates All-Manchester Contest
In the supporting bout, Arthur defeated Bradley Rea by split-decision in a excellent Manchester vs Manchester battle to secure the continental championship.
The stadium fell silent for a profoundly moving homage before the fight as a bell ceremony tolled in honor of local hero Ricky Hatton, who passed away at age 46 in September.
It was the first major televised boxing event in the location since Hatton's death. As the concluding sound faded, the silence gave way to a shout of "there's just one Ricky Hatton".
The fighting inside the ropes reflected the emotion. Arthur was knocked down by a significant blow flush on the face in the early round, standing uncertainly to his footing on the eight count.
But the traditional red attire of the fighter soon advanced again. On the opposite side, Rea - in rival City blue - was marked up early, heavy swelling appearing under his orbital area as the fighters traded powerful shots.
Rea, 27, connected with strong blows but the champion, 34, matched the intensity with effective uppercuts as the action hardly diminished over twelve rounds.
Arthur edged the result with official scores of draw, close margin and 115-112, but each competitor were persistent and courageous, everything their inspiration the legend embodied.
