UK light-heavyweight Buatsi made a winning yet disappointing start to life under new promoters with a decision win over Parker in Manchester.
The Olympic bronze medallist's work rate appeared to earn him a split decision in a competitive ten-round fight at the Manchester venue.
But Parker found success, delivering effective single shots in a fight where the styles never quite clicked, with frequent clinching disrupting the fight.
The officials scored it 96-94 to the winner, with the final judge a 95-95 draw.
"It wasn't my finest performance but what defines champions? They prevail," Buatsi said in the ring as he was faced boos from the spectators.
The Londoner got back to winning after his initial pro loss - an memorable fight with his previous opponent in last winter - to secure a twentieth professional win.
Some ringside observers believed Parker, 31, deserved the victory but the contender suffered the second loss of his 28-match professional journey.
The Champion Claims Triumph in Uneventful Match
His career development has stalled in recent times through management issues and long spells of lack of fights.
Previously predicted as the future of the British fight scene, he now finds himself trying to achieve expectations - perhaps in what seems like a ultimate opportunity - under recent support.
He started carefully, with neither fighter willing to be aggressive in a cagey opening few rounds.
Parker, whose single earlier setback came at the lower weight class against John Ryder in 2022 after damaging his hand mid-fight, was competing in only his second appearance at the higher weight class.
By the fight's midpoint, Parker had made it competitive, causing problems for the favorite, who worked behind a tight guard to edge the cleaner periods.
The spectators' acceptance diminished amid constant clinching, with the challenger going down multiple occasions during clinches.
The underdog closed well, but several stanzas were challenging to assess. However, former world champion the boxing analyst - working with the broadcast network - had it significantly in favor for the challenger, awarding Buatsi just only one stanza.
The victor progresses, but with the caliber of power puncher David Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the path to championship success - based on this performance - still appears challenging.
Ricky Hatton Tribute Fuels Manchester Derby Contest
In the co-main event, Arthur overcame Bradley Rea by close verdict in a excellent all-Manchester contest to win the European light-heavyweight belt.
The arena fell silent for a extremely emotional homage before the fight as 10 bells rang in honor of local hero Ricky Hatton, who passed away 46 years old in recently.
It was the first major televised fight night in the location since the passing of Hatton. As the last bell disappeared, the quiet became a chant of "only one Ricky Hatton".
The competition within the ring matched the sentiment. Arthur was sent to the canvas by a powerful punch directly to the jaw in the initial stage, standing uncertainly to his footing on the referee's count.
But the traditional red equipment of Arthur soon came forward again. From the other corner, the opponent - in competitive blue - was marked up early, noticeable damage forming under his orbital area as the duo landed hard punches.
Rea, 27, delivered effective strikes but the champion, thirty-four, responded in kind with effective uppercuts as the intensity hardly diminished over twelve rounds.
Arthur edged the verdict with official scores of even score, narrow lead and clearer margin, but the two athletes were relentless and brave, exactly what their hero Hatton embodied.
