UK light heavyweight Buatsi earned a win yet unimpressive start to his career under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic medal winner's activity level appeared to earn him a majority decision in a tight ten-round bout at the Manchester venue.
But Parker landed significant shots, connecting with powerful individual punches in a fight where the styles didn't complement each other, with constant grabbing marring the action.
Two judges judged it 96-94 to the winner, with the remaining official a 95-95 draw.
"It wasn't my best showing but what defines champions? They win," the victor stated in the ring as he was met by negative reactions from the spectators.
The Londoner got back to winning after his initial pro loss - an thrilling contest with his previous opponent in February - to earn a twentieth professional win.
Several boxing experts believed Parker, thirty-one, should have been awarded the win but the British boxer endured the second defeat of his 28-bout career.
The Champion Earns Victory in Dull Match
The fighter's advancement has plateaued in recent years through management issues and long spells of inactivity.
Once tipped as the next star of British boxing, he now finds himself trying to realize his potential - possibly in what feels like a final roll of the dice - under new promotional backing.
He opened tentatively, with both competitors willing to take risks in a tentative initial rounds.
The challenger, whose single earlier setback came at the lower weight class against John Ryder in 2022 after breaking his hand mid-fight, was participating in his second bout at light heavyweight.
By the halfway stage, the underdog had kept it close, causing problems for the favorite, who worked behind a solid defense to edge the cleaner periods.
The spectators' acceptance diminished during frequent holding, with the challenger slipping to the canvas several times during clinches.
Parker closed well, but several stanzas were difficult to score. Nevertheless, ex-champion Carl Frampton - providing analysis for Dazn - had it significantly in favor for the challenger, awarding Buatsi just only one stanza.
The victor advances, but with the likes of heavy hitter David Benavidez, and Russian champion Bivol dominating the weight class, the road to world glory - judging by this showing - still looks steep.
Ricky Hatton Memorial Fuels Local Rivalry Fight
In the supporting bout, Lyndon Arthur overcame Bradley Rea by close verdict in a outstanding Manchester vs Manchester contest to win the European light heavyweight belt.
The arena grew hushed for a extremely emotional tribute before the fight as ten bells rang in memory of the city's native Hatton, who died aged 46 in September.
It was the initial significant broadcast boxing event in the urban center since Hatton's passing. As the last bell disappeared, the stillness gave way to a roar of "there's just one Ricky Hatton".
The fighting during the bout equaled the emotion. Arthur was sent to the canvas by a significant blow squarely on the face in the early round, standing uncertainly to his feet on the count of eight.
But the Manchester United-red shorts of the fighter soon came forward again. Across the ring, the opponent - in opposing blue - was injured early, noticeable damage appearing under his eye as the pair exchanged hard punches.
The challenger, twenty-seven, connected with strong blows but Arthur, 34, responded in kind with stinging uppercuts as the pace seldom decreased over twelve rounds.
The victor narrowly won the verdict with scoring tallies of 114-114, close margin and wider gap, but the two athletes were persistent and full of heart, all the qualities their idol the legend embodied.
