British light heavyweight Buatsi made a winning yet disappointing beginning to his career under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's activity level seemingly secured him a majority decision in a close 10-round bout at the Co-op Live Arena.
But the challenger landed significant shots, connecting with powerful clean blows in a fight where the approaches failed to mesh, with excessive holding spoiling the contest.
The officials judged it 96-94 for to Buatsi, with the final judge a even score.
"It wasn't my best showing but what do champions do? They succeed," Buatsi commented in the ring as he was met by boos from the spectators.
The British fighter got back to winning after his first professional defeat - an memorable fight with his previous opponent in February - to claim a twentieth professional win.
Several boxing experts believed Parker, thirty-one, should have been awarded the win but the British boxer suffered the second loss of his 28-fight career.
The Champion Claims Victory in Dull Affair
His career development has slowed in recent years through promotional disputes and extended periods of inactivity.
Earlier forecast as the next star of British boxing, he now finds himself trying to fulfil that early promise - maybe in what appears to be a ultimate opportunity - under new promotional backing.
He started carefully, with neither fighter ready to be aggressive in a cagey initial rounds.
Parker, whose sole prior loss came at super-middleweight against John Ryder in 2022 after damaging his hand during the bout, was competing in only his second appearance at light-heavyweight.
By the fight's midpoint, Parker had kept it close, causing problems for Buatsi, who worked behind a protective stance to narrowly win the cleaner rounds.
The audience's tolerance wore thin because of frequent holding, with Parker slipping to the canvas various instances during clinches.
The underdog finished the stronger, but several stanzas were hard to judge. Nevertheless, former world champion the boxing analyst - commentating on the broadcast network - had it particularly wide for Parker, scoring for Buatsi just only one stanza.
Buatsi moves on, but with the caliber of hard hitter David Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the path to championship success - based on this performance - still appears challenging.
Ricky Hatton Tribute Inspires Local Rivalry Fight
In the co-main event, Lyndon Arthur defeated Bradley Rea by close verdict in a outstanding local rivalry contest to secure the continental title.
The stadium became quiet for a extremely emotional homage prior to the bout as ten bells sounded in honor of local hero Ricky Hatton, who succumbed 46 years old in September.
It was the premier televised match card in the urban center since Hatton's death. As the concluding sound ended, the silence gave way to a roar of "there's just one Ricky Hatton".
The fighting within the ring matched the feeling. The eventual winner was knocked down by a significant blow flush on the face in the initial stage, getting up unsteadily to his feet on the referee's count.
But the Manchester United-red equipment of the fighter soon pressed ahead again. Across the ring, the opponent - in rival City blue - was injured early, noticeable damage appearing under his eye as the fighters landed punishing blows.
The challenger, twenty-seven, connected with strong blows but Arthur, 34, fought fire with fire with powerful upward punches as the action hardly diminished over twelve rounds.
The victor edged the decision with scoring tallies of 114-114, 115-113 and 115-112, but the two athletes were determined and full of heart, all the qualities their hero Hatton embodied.
