UK light-heavyweight Buatsi earned a win yet disappointing start to his career under recent management with a unanimous decision over Parker in Manchester.
The Olympic medal winner's work rate appeared to earn him a judges' verdict in a close ten-round bout at the boxing arena.
But Parker landed significant shots, landing some telling clean blows in a fight where the techniques never quite clicked, with frequent clinching marring the fight.
The scoring judges had it 96-94 in favor to the winner, with the final judge a 95-95.
"It wasn't my best display but how do champions respond? They win," the victor stated in the ringside as he was greeted with boos from the audience.
The British fighter got back to winning after his initial pro loss - an memorable fight with Callum Smith in last winter - to claim a twentieth professional win.
Various analysts thought the challenger, thirty-one, deserved the win but the Derbyshire fighter experienced the second setback of his 28-match career.
The Champion Earns Win in Dull Encounter
The fighter's advancement has slowed in recent years through contract problems and extended periods of lack of fights.
Previously predicted as the next star of the British fight scene, he now finds himself trying to realize his potential - possibly in what feels like a last chance - under fresh management.
He began cautiously, with the two boxers prepared to overcommit in a cagey early stages.
The challenger, whose sole prior loss came at the lower weight class against his former opponent in 2022 after suffering a hand injury mid-fight, was making just his second bout at the higher weight class.
By the halfway stage, Parker had made it competitive, making life awkward for Buatsi, who worked behind a protective stance to barely take the cleaner stanzas.
The audience's tolerance decreased amid constant clinching, with Parker going down several times during clinches.
The underdog ended stronger, but multiple periods were difficult to score. Although, ex-champion the boxing analyst - providing analysis for Dazn - had it significantly in favor for the challenger, scoring for Buatsi just only one stanza.
Buatsi advances, but with the caliber of hard hitter David Benavidez, and Russian champion Bivol dominating the weight class, the journey to titles - on this evidence - still looks steep.
Ricky Hatton Homage Inspires Manchester Derby Fight
In the secondary fight, Arthur overcame Bradley Rea by divided decision in a outstanding Manchester vs Manchester battle to secure the continental championship.
The stadium fell silent for a deeply poignant tribute ahead of the contest as ten bells sounded in memory of local hero Ricky Hatton, who succumbed at age 46 in last month.
It was the first major televised boxing event in the city since Hatton's death. As the last bell faded, the quiet became a roar of "only one Hatton".
The fighting inside the ropes reflected the emotion. Arthur was knocked down by a left hook directly to the face in the initial stage, getting up unsteadily to his position on the eight count.
But the red-colored shorts of the fighter soon pressed ahead again. Across the ring, the opponent - in rival City blue - was marked up early, noticeable damage forming under his face as the duo traded punishing blows.
The challenger, 27, connected with strong blows but the champion, 34, fought fire with fire with effective uppercuts as the action seldom decreased over twelve rounds.
Arthur narrowly won the decision with scoring tallies of even score, narrow lead and wider gap, but the two athletes were persistent and full of heart, everything their idol the legend represented.
