British light-heavyweight Buatsi secured a victory yet unimpressive start to his career under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's work rate seemingly secured him a split decision in a tight ten-round bout at the boxing arena.
But Parker found success, delivering effective clean blows in a match where the techniques failed to mesh, with excessive holding spoiling the fight.
Two judges had it 96-94 to the winner, with the third a 95-95.
"It wasn't my finest performance but what do champions do? They prevail," the victor said in the arena as he was faced negative reactions from the spectators.
The Londoner returned to winning ways after his initial pro loss - an thrilling contest with Callum Smith in February - to claim a twentieth professional win.
Some ringside observers thought the challenger, 31, should have been awarded the victory but the contender endured the second loss of his 28-bout professional journey.
Buatsi Earns Triumph in Uneventful Encounter
The fighter's advancement has stalled in recent times through management issues and considerable time of lack of fights.
Once tipped as the next star of the British fight scene, he now is positioned trying to realize his potential - perhaps in what seems like a last chance - under fresh management.
He opened tentatively, with neither fighter willing to take risks in a calculated early stages.
The challenger, whose only previous defeat came at super-middleweight against John Ryder in 2022 after breaking his hand during the bout, was making just his second bout at the higher weight class.
By the fight's midpoint, the underdog had stayed in contention, making life awkward for Buatsi, who operated with a solid defense to barely take the more effective stanzas.
The audience's tolerance wore thin because of regular grabbing, with the challenger falling to the floor several times during clinches.
Parker closed well, but multiple periods were challenging to assess. Although, ex-champion Carl Frampton - commentating on Dazn - had it significantly in favor for Parker, awarding Buatsi just only one stanza.
Buatsi progresses, but with the caliber of hard hitter David Benavidez, and Russian star Bivol leading the category, the path to championship success - on this evidence - still appears challenging.
Hatton Memorial Motivates Manchester Derby Battle
In the supporting bout, Arthur defeated Bradley Rea by close verdict in a terrific local rivalry contest to capture the European light-heavyweight belt.
The arena grew hushed for a deeply poignant memorial prior to the bout as a bell ceremony sounded in remembrance of local hero Hatton, who died at age 46 in September.
It was the first major televised fight night in the location since Hatton's death. As the last bell disappeared, the quiet gave way to a chant of "only one Ricky Hatton".
The fighting within the ring equaled the emotion. Arthur was knocked down by a powerful punch directly to the chin in the second round, rising shakily to his feet on the eight count.
But the traditional red shorts of the fighter soon pressed ahead again. On the opposite side, Rea - in rival City blue - was injured soon, heavy swelling developing under his orbital area as the pair exchanged powerful shots.
Rea, 27, connected with strong blows but Arthur, 34, responded in kind with stinging uppercuts as the intensity rarely dropped over the full distance.
Arthur barely secured the verdict with judges scorecards of 114-114, 115-113 and clearer margin, but each competitor were relentless and full of heart, all the qualities their hero Hatton represented.
