British light heavyweight Buatsi secured a victory yet underwhelming beginning to life 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 competitive 10-round bout at the Manchester venue.
But Parker found success, connecting with powerful individual punches in a match where the styles didn't complement each other, with excessive holding marring the fight.
Two judges judged it 96-94 to the winner, with the third a even score.
"It wasn't my greatest showing but what defines champions? They win," the victor commented in the ringside as he was greeted with boos from the audience.
The Londoner returned to winning ways after his first professional defeat - an epic battle with his previous opponent in last winter - to claim a 20th career victory.
Some ringside observers believed Parker, thirty-one, merited the victory but the Derbyshire fighter suffered the second setback of his 28-bout career.
Buatsi Secures Win in Dull Affair
The fighter's advancement has slowed in recent times through contract problems and long spells of lack of fights.
Once tipped as the next star of the British fight scene, he now finds himself trying to achieve expectations - maybe in what feels like a last chance - under new promotional backing.
He began cautiously, with both competitors ready 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 appearance at the higher weight class.
By the halfway stage, Parker had stayed in contention, causing problems for Buatsi, who utilized a protective stance to narrowly win the more effective periods.
The spectators' acceptance wore thin amid constant clinching, with the challenger going down various instances during grappling.
Parker finished the stronger, but multiple periods were challenging to assess. Nevertheless, ex-champion Carl Frampton - working with Dazn - had it strongly leaning for the challenger, awarding Buatsi just only one stanza.
Buatsi moves on, but with the likes of power puncher Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the road to world glory - judging by this showing - still appears challenging.
Ricky Hatton Tribute Fuels Manchester Derby Battle
In the co-main event, Arthur defeated Rea by split-decision in a outstanding local rivalry battle to win the European light heavyweight title.
The venue became quiet for a deeply poignant homage before the fight as ten bells tolled in remembrance of the city's native Hatton, who died 46 years old in September.
It was the first major televised match card in the location since Hatton's death. As the concluding sound disappeared, the quiet gave way to a chant of "there's just one Hatton".
The competition during the bout matched the feeling. The eventual winner was sent to the canvas by a powerful punch flush on the chin in the initial stage, getting up unsteadily to his feet on the count of eight.
But the traditional red equipment of Arthur soon advanced again. From the other corner, Rea - in competitive blue - was damaged quickly, heavy swelling forming under his face as the fighters traded powerful shots.
Rea, twenty-seven, connected with strong blows but the champion, thirty-four, responded in kind with effective uppercuts as the action rarely dropped over 12 rounds.
Arthur barely secured the result with scoring tallies of draw, narrow lead and 115-112, but both fighters were persistent and brave, exactly what their hero Hatton embodied.
