UK light heavyweight Buatsi made a winning yet disappointing beginning to his career under new promoters with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's work rate appeared to earn him a judges' verdict in a close 10-round bout at the Co-op Live Arena.
But Parker had his moments, landing some telling individual punches in a contest where the techniques failed to mesh, with frequent clinching spoiling the action.
Two judges judged it 96-94 in favor to Buatsi, with the third a 95-95.
"It wasn't my finest performance but how do champions respond? They succeed," Buatsi commented in the arena as he was faced boos from the crowd.
The Londoner bounced back after his first professional defeat - an memorable fight with his previous opponent in February - to earn a 20th pro success.
Some ringside observers thought the challenger, 31, merited the win but the Derbyshire fighter endured the second setback of his 28-bout career.
The Champion Earns Triumph in Slow Match
Buatsi's progress has stalled in recent times through contract problems and extended periods of inactivity.
Previously predicted as the rising talent of British boxing, he now is positioned trying to fulfil that early promise - perhaps in what appears to be a last chance - under new promotional backing.
He started carefully, with both competitors prepared to be aggressive in a cagey early stages.
The challenger, whose only previous defeat came at super-middleweight against his former opponent in 2022 after breaking his hand during the bout, was competing in only his second bout at light-heavyweight.
By the middle rounds, Parker had kept it close, causing problems for Buatsi, who utilized a tight guard to narrowly win the more effective stanzas.
The audience's tolerance decreased amid frequent holding, with the challenger falling to the floor various instances during close exchanges.
The underdog finished the stronger, but several stanzas were hard to judge. Although, former world champion the boxing analyst - working with Dazn - had it particularly wide for the challenger, scoring for Buatsi just one round.
Buatsi moves on, but with the caliber of hard hitter Benavidez, and Russian star Bivol leading the category, the journey to titles - on this evidence - still seems difficult.
Ricky Hatton Memorial Motivates All-Manchester Battle
In the supporting bout, Arthur beat Bradley Rea by close verdict in a terrific local rivalry contest to win the European light heavyweight championship.
The arena fell silent for a extremely emotional tribute before the fight as 10 bells tolled in memory of Manchester's own Hatton, who passed away at age 46 in recently.
It was the premier televised boxing event in the city since Hatton's passing. As the last bell disappeared, the quiet transitioned to a shout of "there's just one Ricky Hatton".
The action during the bout reflected the sentiment. Arthur was floored by a significant blow squarely on the jaw in the initial stage, rising shakily to his feet on the count of eight.
But the Manchester United-red equipment of the fighter soon came forward again. Across the ring, the opponent - in rival City blue - was damaged soon, noticeable damage developing under his eye as the pair exchanged powerful shots.
The challenger, 27, delivered effective strikes but Arthur, 34, matched the intensity with powerful upward punches as the pace rarely dropped over 12 rounds.
Arthur barely secured the decision with scoring tallies of 114-114, narrow lead and clearer margin, but the two athletes were persistent and full of heart, all the qualities their idol Hatton stood for.
