UK light heavyweight Buatsi made a winning yet disappointing debut to his career under recent management with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's punch output appeared to earn him a judges' verdict in a tight 10-round contest at the Manchester venue.
But Parker had his moments, landing some telling individual punches in a match where the techniques never quite clicked, with excessive holding spoiling the fight.
The scoring judges judged it 96-94 for to the winner, with the final judge a 95-95 draw.
"It wasn't my best showing but how do champions respond? They succeed," Buatsi stated in the ringside as he was greeted with negative reactions from the audience.
The British fighter returned to winning ways after his first professional defeat - an memorable fight with his previous opponent in last winter - to claim a 20th pro success.
Some ringside observers thought the challenger, 31, should have been awarded the decision but the Derbyshire fighter endured the second setback of his 28-fight career.
The Champion Secures Win in Uneventful Encounter
The fighter's advancement has slowed in the past few years through promotional disputes and long spells of inactivity.
Earlier forecast as the next star of the British fight scene, he now stands trying to fulfil that early promise - perhaps in what feels like a ultimate opportunity - under new promotional backing.
He opened tentatively, with both competitors willing to be aggressive in a tentative opening few rounds.
The challenger, whose sole prior loss came at super-middleweight against John Ryder in 2022 after suffering a hand injury mid-fight, was participating in his second fight at the higher weight class.
By the halfway stage, the underdog had kept it close, causing problems for the favorite, who worked behind a solid defense to edge the cleaner periods.
The spectators' acceptance diminished during regular grabbing, with Parker slipping to the canvas several times during clinches.
The underdog closed well, but several stanzas were hard to judge. Nevertheless, ex-champion Carl Frampton - commentating on the broadcast network - had it significantly in favor for the challenger, giving Buatsi just only one stanza.
The victor moves on, but with the caliber of power puncher David Benavidez, and Russian champion Dmitry Bivol leading the category, the path to championship success - judging by this showing - still seems difficult.
Ricky Hatton Homage Motivates Manchester Derby Contest
In the co-main event, Arthur overcame Rea by close verdict in a terrific all-Manchester contest to secure the European light-heavyweight championship.
The stadium grew hushed for a profoundly moving memorial before the fight as a bell ceremony sounded in memory of local hero Ricky Hatton, who died 46 years old in last month.
It was the initial significant broadcast boxing event in the urban center since Hatton's passing. As the concluding sound faded, the silence became a shout of "there's just one Hatton".
The action within the ring equaled the emotion. Arthur was sent to the canvas by a left hook flush on the face in the second round, getting up unsteadily to his feet on the eight count.
But the Manchester United-red attire of the fighter soon came forward again. Across the ring, Rea - in competitive blue - was damaged early, significant bruising appearing under his orbital area as the pair exchanged punishing blows.
The challenger, twenty-seven, delivered effective strikes but the champion, 34, responded in kind with effective uppercuts as the pace hardly diminished over the full distance.
The victor narrowly won the decision with official scores of 114-114, narrow lead and 115-112, but both fighters were determined and full of heart, all the qualities their idol Hatton stood for.
