British light-heavyweight Buatsi secured a victory yet unimpressive start to life under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's activity level appeared to earn him a judges' verdict in a close 10-round bout at the Co-op Live Arena.
But the challenger landed significant shots, landing some telling single shots in a match where the styles failed to mesh, with constant grabbing marring the action.
Two judges scored it 96-94 to Buatsi, with the remaining official a 95-95.
"It wasn't my best performance but what do champions do? They succeed," the victor said in the ringside as he was faced boos from the audience.
The Londoner bounced back after his maiden career setback - an memorable fight with his previous opponent in February - to secure a twentieth professional win.
Various analysts believed Parker, 31, deserved the decision but the British boxer endured the second setback of his 28-fight career.
Buatsi Secures Victory in Uneventful Encounter
Buatsi's progress has stalled in recent years through contract problems and long spells of inactivity.
Once tipped as the rising talent of UK boxing, he now stands trying to fulfil that early promise - possibly in what appears to be a last chance - under fresh management.
He opened tentatively, with the two boxers willing to be aggressive in a tentative early stages.
The challenger, whose sole prior loss came at the lower weight class against John Ryder in 2022 after breaking his hand mid-fight, was making just his second appearance at light heavyweight.
By the halfway stage, Parker had made it competitive, creating difficulties for the favorite, who operated with a protective stance to edge the cleaner rounds.
The spectators' acceptance wore thin amid constant clinching, with the challenger going down various instances during close exchanges.
The underdog closed well, but multiple periods were difficult to score. Although, previous titleholder the boxing analyst - commentating on Dazn - had it particularly wide for Parker, awarding Buatsi just only one stanza.
Buatsi advances, but with the likes of heavy hitter David Benavidez, and Eastern European fighter Bivol leading the category, the road to world glory - on this evidence - still appears challenging.
Ricky Hatton Tribute Fuels Manchester Derby Battle
In the supporting bout, Arthur beat Bradley Rea by close verdict in a terrific local rivalry fight to win the continental title.
The stadium became quiet for a profoundly moving tribute ahead of the contest as a bell ceremony tolled in memory of Manchester's own Ricky Hatton, who passed away aged 46 in last month.
It was the premier televised boxing event in the city since Hatton's passing. As the last bell ended, the stillness gave way to a shout of "there's just one Ricky Hatton".
The fighting during the bout equaled the emotion. The eventual winner was floored by a powerful punch squarely on the chin in the initial stage, getting up unsteadily to his footing on the eight count.
But the traditional red shorts of Arthur soon came forward again. On the opposite side, Rea - in competitive blue - was injured quickly, heavy swelling forming under his orbital area as the duo landed powerful shots.
Rea, 27, landed power punches but Arthur, thirty-four, responded in kind with powerful upward punches as the intensity rarely dropped over the full distance.
Arthur narrowly won the result with judges scorecards of draw, 115-113 and 115-112, but both fighters were relentless and brave, exactly what their idol the legend embodied.
