UK light heavyweight Buatsi secured a victory yet disappointing start to life under new promoters with a unanimous decision over Parker in Manchester.
The Olympic medal winner's activity level seemingly secured him a majority decision in a tight ten-round fight at the boxing arena.
But the challenger had his moments, delivering effective individual punches in a contest where the styles failed to mesh, with frequent clinching disrupting the fight.
Two judges scored it 96-94 in favor to Buatsi, with the remaining official a even score.
"It wasn't my greatest showing but what defines champions? They prevail," Buatsi said in the ringside as he was faced boos from the spectators.
The Londoner bounced back after his initial pro loss - an thrilling contest with Callum Smith in last winter - to claim a 20th career victory.
Various analysts felt the challenger, thirty-one, should have been awarded the win but the Derbyshire fighter experienced the second loss of his 28-fight professional journey.
The Champion Earns Victory in Slow Encounter
The fighter's advancement has plateaued in recent years through contract problems and extended periods of inactivity.
Once tipped as the future of UK boxing, he now stands trying to achieve expectations - perhaps in what seems like a final roll of the dice - under recent support.
He started carefully, with the two boxers willing to be aggressive in a tentative opening few rounds.
The challenger, whose only previous defeat came at super-middleweight against his former opponent in 2022 after damaging his hand during the bout, was making just his second bout at light heavyweight.
By the middle rounds, Parker had kept it close, making life awkward for Buatsi, who operated with a protective stance to edge the cleaner stanzas.
The audience's tolerance wore thin amid regular grabbing, with the challenger going down multiple occasions during close exchanges.
Parker ended stronger, but many rounds were difficult to score. Although, ex-champion Carl Frampton - working with Dazn - had it strongly leaning for Parker, giving Buatsi just one round.
The victor progresses, but with the likes of hard hitter David Benavidez, and Russian star Bivol dominating the weight class, the journey to titles - judging by this showing - still seems difficult.
Hatton Homage Fuels All-Manchester Fight
In the supporting bout, Arthur overcame Bradley Rea by divided decision in a outstanding local rivalry contest to win the European light-heavyweight belt.
The arena grew hushed for a extremely emotional homage ahead of the contest as ten bells tolled in remembrance of local hero Hatton, who passed away at age 46 in recently.
It was the premier televised fight night in the city since the passing of Hatton. As the last bell faded, the quiet gave way to a shout of "there's only one Hatton".
The fighting inside the ropes matched the feeling. Arthur was knocked down by a left hook directly to the face in the early round, getting up unsteadily to his footing on the count of eight.
But the red-colored equipment of Arthur soon came forward again. On the opposite side, the opponent - in opposing blue - was injured early, heavy swelling forming under his eye as the pair traded punishing blows.
Rea, 27, connected with strong blows but the champion, thirty-four, matched the intensity with powerful upward punches as the intensity hardly diminished over the full distance.
The victor narrowly won the verdict with scoring tallies of draw, 115-113 and wider gap, but both fighters were determined and courageous, all the qualities their idol the legend embodied.
