UK light heavyweight Buatsi made a winning yet disappointing start 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 majority decision in a close 10-round fight at the Manchester venue.
But the challenger found success, connecting with powerful individual punches in a fight where the approaches didn't complement each other, with frequent clinching marring the contest.
Two judges judged it 96-94 in favor to Buatsi, with the remaining official a 95-95 draw.
"It wasn't my finest showing but how do champions respond? They succeed," Buatsi commented in the ring as he was faced disapproval from the audience.
The British fighter returned to winning ways after his first professional defeat - an memorable fight with Callum Smith in last winter - to claim a twentieth professional win.
Several boxing experts thought Parker, thirty-one, should have been awarded the victory but the contender suffered the second setback of his 28-match career.
The Champion Secures Triumph in Dull Match
Buatsi's progress has stalled in recent years through management issues and long spells of lack of fights.
Previously predicted as the rising talent of British boxing, he now finds himself trying to achieve expectations - possibly in what seems like a last chance - under recent support.
He opened tentatively, with the two boxers prepared to take risks in a cagey initial rounds.
The challenger, whose single earlier setback came at super middleweight against his former opponent in 2022 after breaking his hand during the bout, was participating in his second fight at light heavyweight.
By the middle rounds, the underdog had stayed in contention, causing problems for Buatsi, who worked behind a solid defense to narrowly win the more effective stanzas.
The spectators' acceptance wore thin because of constant clinching, with the challenger slipping to the canvas several times during clinches.
The underdog ended stronger, but several stanzas were difficult to score. Nevertheless, ex-champion the boxing analyst - commentating on Dazn - had it strongly leaning for Parker, giving Buatsi just one round.
Buatsi moves on, but with the caliber of power puncher David Benavidez, and Russian champion Bivol ruling the division, the road to world glory - based on this performance - still looks steep.
Hatton Memorial Inspires All-Manchester Battle
In the co-main event, Arthur overcame Bradley Rea by close verdict in a outstanding Manchester vs Manchester fight to secure the continental belt.
The arena grew hushed for a profoundly moving homage before the fight as ten bells sounded in memory of Manchester's own Hatton, who passed away aged 46 in recently.
It was the first major televised match card in the location since the passing of Hatton. As the concluding sound disappeared, the quiet transitioned to a shout of "only one Ricky Hatton".
The action within the ring equaled the emotion. Arthur was floored by a powerful punch squarely on the face in the initial stage, getting up unsteadily to his position on the eight count.
But the traditional red shorts of the fighter soon came forward again. From the other corner, Rea - in rival City blue - was damaged quickly, noticeable damage appearing under his eye as the fighters traded powerful shots.
The challenger, twenty-seven, landed power punches but the champion, thirty-four, matched the intensity with effective uppercuts as the action rarely dropped over twelve rounds.
The victor barely secured the result with official scores of draw, 115-113 and wider gap, but both fighters were persistent and full of heart, exactly what their hero Hatton represented.
