British light-heavyweight Buatsi made a winning yet disappointing start to his career under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic bronze medallist's work rate seemingly secured him a majority decision in a competitive ten-round fight at the Manchester venue.
But Parker landed significant shots, landing some telling individual punches in a match where the techniques failed to mesh, with frequent clinching disrupting the contest.
The scoring judges scored it 96-94 for to the winner, with the third a 95-95 draw.
"It wasn't my best showing but what defines champions? They succeed," the victor commented in the arena as he was met by boos from the crowd.
The British fighter bounced back after his first professional defeat - an thrilling contest with Callum Smith in last winter - to earn a 20th career victory.
Some ringside observers thought the challenger, thirty-one, deserved the decision but the British boxer experienced the second setback of his 28-fight career.
The Champion Secures Victory in Uneventful Encounter
The fighter's advancement has plateaued in the past few years through contract problems and considerable time of inactivity.
Previously predicted as the next star of the British fight scene, he now finds himself trying to achieve expectations - perhaps in what feels like a ultimate opportunity - under recent support.
He began cautiously, with both competitors willing to take risks in a tentative early stages.
Parker, whose only previous defeat came at the lower weight class against John Ryder in 2022 after damaging his hand in the contest, was making just his second bout at the higher weight class.
By the middle rounds, Parker had kept it close, creating difficulties for Buatsi, who operated with a protective stance to barely take the more effective stanzas.
The crowd's patience wore thin during frequent holding, with Parker slipping to the canvas several times during close exchanges.
Parker closed well, but many rounds were challenging to assess. Nevertheless, ex-champion the boxing analyst - working with the broadcast network - had it significantly in favor for Parker, awarding Buatsi just only one stanza.
The victor advances, but with the presence of heavy hitter David Benavidez, and Russian star Dmitry Bivol ruling the division, the road to world glory - on this evidence - still looks steep.
Hatton Tribute Fuels All-Manchester Fight
In the secondary fight, Lyndon Arthur defeated Bradley Rea by divided decision in a terrific Manchester vs Manchester fight to capture the European light-heavyweight title.
The arena became quiet for a deeply poignant homage ahead of the contest as a bell ceremony sounded in memory of Manchester's own Ricky Hatton, who died 46 years old in recently.
It was the premier televised boxing event in the city since the passing of Hatton. As the concluding sound faded, the quiet became a roar of "only one Hatton".
The fighting within the ring equaled the sentiment. Arthur was knocked down by a powerful punch directly to the chin in the initial stage, rising shakily to his footing on the eight count.
But the Manchester United-red equipment of Arthur soon came forward again. On the opposite side, the opponent - in competitive blue - was marked up quickly, significant bruising forming under his eye as the pair exchanged hard punches.
Rea, 27, delivered effective strikes but Arthur, 34, matched the intensity with stinging uppercuts as the pace rarely dropped over twelve rounds.
The victor narrowly won the verdict with scoring tallies of draw, narrow lead and wider gap, but each competitor were determined and courageous, all the qualities their inspiration the legend embodied.
