British light-heavyweight Buatsi earned a win yet underwhelming start to life under new promoters with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output likely gained him a judges' verdict in a tight ten-round bout at the Co-op Live Arena.
But the challenger had his moments, delivering effective clean blows in a fight where the approaches failed to mesh, with excessive holding spoiling the action.
Two judges scored it 96-94 for to the winner, with the final judge a 95-95 draw.
"It wasn't my best showing but how do champions respond? They succeed," the victor said in the ring as he was greeted with disapproval from the audience.
The Londoner returned to winning ways after his initial pro loss - an epic battle with Callum Smith in last winter - to claim a 20th career victory.
Some ringside observers believed Parker, 31, merited the victory but the Derbyshire fighter suffered the second setback of his 28-match professional journey.
The Champion Claims Triumph in Slow Match
Buatsi's progress has plateaued in the past few years through management issues and extended periods of lack of fights.
Previously predicted as the rising talent of British boxing, he now stands trying to achieve expectations - possibly in what seems like a last chance - under recent support.
He began cautiously, with the two boxers prepared to be aggressive in a cagey opening few rounds.
Parker, whose only previous defeat came at super middleweight against his former opponent in 2022 after breaking his hand mid-fight, was competing in only his second fight at the higher weight class.
By the middle rounds, Parker had made it competitive, making life awkward for the favorite, who worked behind a protective stance to edge the better rounds.
The audience's tolerance wore thin amid regular grabbing, with Parker going down multiple occasions during clinches.
The underdog closed well, but multiple periods were hard to judge. Although, previous titleholder Carl Frampton - commentating on the broadcast network - had it significantly in favor for Parker, scoring for Buatsi just one round.
Buatsi advances, but with the caliber of heavy hitter David Benavidez, and Eastern European fighter Bivol ruling the division, the path to championship success - judging by this showing - still looks steep.
Hatton Homage Motivates Local Rivalry Battle
In the supporting bout, Lyndon Arthur overcame Bradley Rea by split-decision in a terrific local rivalry battle to secure the European light heavyweight championship.
The stadium fell silent for a extremely emotional homage prior to the bout as 10 bells tolled in honor of local hero Ricky Hatton, who succumbed 46 years old in last month.
It was the first major televised match card in the urban center since the passing of Hatton. As the final chime faded, the stillness became a roar of "only one Hatton".
The fighting inside the ropes equaled the feeling. Arthur was floored by a powerful punch flush on the face in the second round, standing uncertainly to his footing on the count of eight.
But the Manchester United-red shorts of Arthur soon advanced again. Across the ring, the opponent - in opposing blue - was marked up early, heavy swelling appearing under his orbital area as the pair traded powerful shots.
Rea, 27, delivered effective strikes but Arthur, 34, fought fire with fire with stinging uppercuts as the action rarely dropped over 12 rounds.
Arthur narrowly won the decision with official scores of 114-114, close margin and clearer margin, but both fighters were persistent and courageous, everything their hero Hatton embodied.
