UK light-heavyweight Buatsi earned a win yet disappointing start to his career under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic bronze medallist's punch output appeared to earn him a judges' verdict in a close ten-round bout at the Manchester venue.
But the challenger landed significant shots, landing some telling individual punches in a match where the techniques never quite clicked, with constant grabbing marring the contest.
Two judges scored it 96-94 for to the winner, with the final judge a 95-95 draw.
"It wasn't my greatest showing but what do champions do? They win," the victor said in the ring as he was met by disapproval from the spectators.
The British fighter bounced back after his maiden career setback - an epic battle with Callum Smith in February - to secure a twentieth professional win.
Several boxing experts believed the challenger, thirty-one, deserved the win but the British boxer suffered the second setback of his 28-bout professional journey.
Buatsi Earns Triumph in Dull Encounter
The fighter's advancement has plateaued in recent times through management issues and extended periods of lack of fights.
Earlier forecast as the future of the British fight scene, he now stands trying to achieve expectations - maybe in what seems like a last chance - under recent support.
He began cautiously, with both competitors ready to be aggressive in a cagey initial rounds.
The challenger, whose sole prior loss came at super-middleweight against John Ryder in 2022 after suffering a hand injury in the contest, was participating in his second appearance at light-heavyweight.
By the middle rounds, Parker had stayed in contention, making life awkward for Buatsi, who utilized a solid defense to edge the cleaner stanzas.
The audience's tolerance diminished because of constant clinching, with Parker going down multiple occasions during close exchanges.
Parker finished the stronger, but several stanzas were challenging to assess. Although, previous titleholder the boxing analyst - providing analysis for Dazn - had it significantly in favor for the challenger, scoring for Buatsi just one round.
Buatsi moves on, but with the likes of heavy hitter Benavidez, and Russian champion Bivol leading the category, the path to championship success - based on this performance - still appears challenging.
Ricky Hatton Homage Fuels Local Rivalry Fight
In the secondary fight, Arthur beat Bradley Rea by split-decision in a outstanding all-Manchester contest to capture the European light heavyweight title.
The stadium became quiet for a deeply poignant homage ahead of the contest as a bell ceremony rang in remembrance of Manchester's own Ricky Hatton, who passed away at age 46 in last month.
It was the premier televised match card in the city since Hatton's death. As the final chime disappeared, the silence became a chant of "there's only one Hatton".
The action during the bout matched the sentiment. Arthur was floored by a significant blow flush on the face in the early round, standing uncertainly to his feet on the eight count.
But the traditional red attire of Arthur soon came forward again. From the other corner, Rea - in opposing blue - was damaged soon, noticeable damage developing under his face as the duo traded punishing blows.
The challenger, 27, landed power punches but Arthur, 34, matched the intensity with effective uppercuts as the intensity rarely dropped over 12 rounds.
The victor narrowly won the decision with judges scorecards of 114-114, 115-113 and wider gap, but each competitor were determined and courageous, all the qualities their idol Hatton stood for.
