UK light-heavyweight Buatsi made a winning yet disappointing beginning to his career under new promoters with a decision win over Parker in Manchester.
The Olympic medal winner's punch output appeared to earn him a judges' verdict in a tight ten-round contest at the Manchester venue.
But Parker landed significant shots, landing some telling single shots in a fight where the styles never quite clicked, with frequent clinching spoiling the fight.
The officials scored it 96-94 in favor to the winner, with the final judge a 95-95.
"It wasn't my finest showing but what do champions do? They win," Buatsi stated in the ring as he was greeted with boos from the audience.
The British fighter got back to winning after his first professional defeat - an memorable fight with Callum Smith in February - to earn a twentieth professional win.
Several boxing experts thought the challenger, 31, should have been awarded the decision but the British boxer experienced the second loss of his 28-match career.
Buatsi Secures Triumph in Dull Match
His career development has plateaued in recent times through contract problems and considerable time of inactivity.
Previously predicted as the next star of UK boxing, he now finds himself trying to achieve expectations - possibly in what feels like a ultimate opportunity - under fresh management.
He opened tentatively, with neither fighter prepared to be aggressive in a cagey opening few rounds.
Parker, whose only previous defeat came at super-middleweight against John Ryder in 2022 after breaking his hand in the contest, was participating in his second bout at the higher weight class.
By the halfway stage, the underdog had made it competitive, creating difficulties for the favorite, who utilized a solid defense to barely take the cleaner stanzas.
The spectators' acceptance wore thin because of constant clinching, with Parker falling to the floor various instances during clinches.
The underdog ended stronger, but multiple periods were hard to judge. However, former world champion Carl Frampton - commentating on the broadcast network - had it strongly leaning for the challenger, scoring for Buatsi just one round.
The victor progresses, but with the likes of heavy hitter David Benavidez, and Russian champion Bivol dominating the weight class, the journey to titles - based on this performance - still looks steep.
Ricky Hatton Tribute Motivates Local Rivalry Contest
In the secondary fight, Lyndon Arthur beat Bradley Rea by divided decision in a terrific Manchester vs Manchester fight to win the European light heavyweight title.
The stadium grew hushed for a deeply poignant memorial ahead of the contest as ten bells sounded in memory of Manchester's own Ricky Hatton, who succumbed aged 46 in last month.
It was the premier televised match card in the urban center since the passing of Hatton. As the last bell disappeared, the quiet gave way to a shout of "there's only one Hatton".
The fighting during the bout matched the emotion. Arthur was knocked down by a powerful punch flush on the face in the initial stage, standing uncertainly to his footing on the referee's count.
But the red-colored attire of Arthur soon advanced again. Across the ring, the opponent - in rival City blue - was marked up soon, noticeable damage appearing under his face as the pair exchanged powerful shots.
Rea, twenty-seven, connected with strong blows but Arthur, thirty-four, matched the intensity with stinging uppercuts as the pace hardly diminished over 12 rounds.
The victor edged the verdict with judges scorecards of draw, narrow lead and 115-112, but each competitor were determined and courageous, everything their inspiration Hatton embodied.
