UK light-heavyweight Buatsi secured a victory yet underwhelming debut to life under recent management with a points victory over Parker in Manchester.
The Olympic medal winner's punch output seemingly secured him a majority decision in a competitive ten-round fight at the Co-op Live Arena.
But Parker landed significant shots, delivering effective individual punches in a fight where the styles failed to mesh, with frequent clinching spoiling the fight.
Two judges scored it 96-94 for to the winner, with the remaining official a even score.
"It wasn't my finest performance but what do champions do? They prevail," Buatsi stated in the ring as he was greeted with disapproval from the crowd.
The British fighter bounced back after his first professional defeat - an epic battle with his previous opponent in last winter - to secure a 20th career victory.
Several boxing experts felt Parker, thirty-one, should have been awarded the decision but the British boxer suffered the second defeat of his 28-bout career.
Buatsi Secures Triumph in Uneventful Encounter
The fighter's advancement has slowed in recent years through management issues and long spells of inactivity.
Earlier forecast as the next star of UK boxing, he now stands trying to fulfil that early promise - possibly in what feels like a final roll of the dice - under recent support.
He began cautiously, with neither fighter prepared to overcommit in a calculated opening few rounds.
The challenger, whose single earlier setback came at super-middleweight against his former opponent in two years ago after suffering a hand injury during the bout, was competing in only his second bout at light-heavyweight.
By the middle rounds, Parker had made it competitive, making life awkward for the favorite, who utilized a tight guard to edge the more effective rounds.
The audience's tolerance decreased during constant clinching, with the challenger falling to the floor multiple occasions during grappling.
Parker finished the stronger, but multiple periods were challenging to assess. However, previous titleholder Carl Frampton - commentating on the broadcast network - had it significantly in favor for Parker, giving Buatsi just a single round.
The victor progresses, but with the presence of power puncher David Benavidez, and Russian star Dmitry Bivol leading the category, the road to world glory - based on this performance - still seems difficult.
Ricky Hatton Memorial Fuels Local Rivalry Battle
In the co-main event, Arthur defeated Bradley Rea by split-decision in a outstanding Manchester vs Manchester fight to win the European light heavyweight championship.
The stadium fell silent for a extremely emotional homage prior to the bout as ten bells tolled in memory of local hero Hatton, who succumbed 46 years old in September.
It was the premier televised match card in the urban center since Hatton's passing. As the final chime disappeared, the stillness transitioned to a shout of "there's just one Ricky Hatton".
The action within the ring equaled the emotion. Arthur was sent to the canvas by a significant blow flush on the jaw in the initial stage, rising shakily to his position on the count of eight.
But the red-colored equipment of the fighter soon pressed ahead again. Across the ring, Rea - in competitive blue - was injured early, heavy swelling appearing under his face as the pair landed powerful shots.
The challenger, 27, delivered effective strikes but the champion, 34, matched the intensity with powerful upward punches as the pace rarely dropped over 12 rounds.
Arthur barely secured the verdict with judges scorecards of 114-114, close margin and 115-112, but each competitor were determined and courageous, everything their idol Hatton embodied.
