UK light-heavyweight Buatsi secured a victory yet unimpressive debut to life under new promoters with a unanimous decision over Parker in Manchester.
The Olympic medal winner's punch output appeared to earn him a majority decision in a competitive 10-round contest at the boxing arena.
But the challenger found success, delivering effective clean blows in a contest where the approaches didn't complement each other, with constant grabbing disrupting the contest.
The officials had it 96-94 to Buatsi, with the remaining official a 95-95.
"It wasn't my greatest performance but what defines champions? They succeed," Buatsi stated in the ring as he was greeted with boos from the spectators.
The Londoner bounced back after his first professional defeat - an thrilling contest with his previous opponent in February - to claim a twentieth professional win.
Various analysts felt Parker, thirty-one, deserved the victory but the British boxer endured the second defeat of his 28-match career.
The Champion Claims Victory in Slow Match
The fighter's advancement has slowed in recent years through promotional disputes and extended periods of inactivity.
Previously predicted as the future of the British fight scene, he now is positioned trying to realize his potential - maybe in what appears to be a ultimate opportunity - under new promotional backing.
He began cautiously, with both competitors prepared to overcommit in a cagey initial rounds.
Parker, whose single earlier setback came at the lower weight class against John Ryder in two years ago after damaging his hand during the bout, was competing in only his second appearance at the higher weight class.
By the halfway stage, the underdog had stayed in contention, making life awkward for Buatsi, who operated with a solid defense to barely take the cleaner periods.
The audience's tolerance wore thin amid constant clinching, with Parker going down various instances during close exchanges.
Parker finished the stronger, but multiple periods were challenging to assess. Although, ex-champion Carl Frampton - providing analysis for Dazn - had it strongly leaning for Parker, giving Buatsi just only one stanza.
Buatsi progresses, but with the likes of hard hitter Benavidez, and Russian star Dmitry Bivol ruling the division, the path to championship success - judging by this showing - still appears challenging.
Ricky Hatton Homage Motivates Manchester Derby Battle
In the secondary fight, Arthur defeated Bradley Rea by close verdict in a outstanding all-Manchester fight to secure the continental championship.
The venue fell silent for a extremely emotional homage prior to the bout as ten bells rang in remembrance of local hero Hatton, who died 46 years old in recently.
It was the premier televised boxing event in the location since the passing of Hatton. As the concluding sound ended, the stillness transitioned to a roar of "only one Hatton".
The competition during the bout reflected the emotion. The eventual winner was floored by a powerful punch squarely on the face in the early round, getting up unsteadily to his feet on the count of eight.
But the red-colored attire of the fighter soon came forward again. Across the ring, the opponent - in rival City blue - was damaged soon, heavy swelling forming under his face as the pair landed hard punches.
Rea, twenty-seven, delivered effective strikes but Arthur, thirty-four, responded in kind with powerful upward punches as the intensity rarely dropped over twelve rounds.
Arthur barely secured the verdict with scoring tallies of 114-114, close margin and 115-112, but the two athletes were persistent and brave, all the qualities their inspiration Hatton embodied.
