UK light-heavyweight Buatsi made a winning yet disappointing start to his career under new promoters with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's activity level likely gained him a majority decision in a close ten-round bout at the Manchester venue.
But the challenger found success, delivering effective single shots in a contest where the styles didn't complement each other, with constant grabbing spoiling the contest.
The officials scored it 96-94 for to Buatsi, with the third a 95-95 draw.
"It wasn't my best display but how do champions respond? They win," the victor commented in the ringside as he was faced negative reactions from the audience.
The British fighter returned to winning ways after his maiden career setback - an thrilling contest with his previous opponent in last winter - to claim a twentieth professional win.
Various analysts thought the challenger, thirty-one, should have been awarded the victory but the Derbyshire fighter suffered the second defeat of his 28-bout career.
The Champion Earns Victory in Uneventful Encounter
His career development has plateaued in recent years through management issues and extended periods of inactivity.
Previously predicted as the next star of the British fight scene, he now finds himself trying to fulfil that early promise - possibly in what feels like a last chance - under recent support.
He began cautiously, with neither fighter willing to take risks in a tentative initial rounds.
Parker, whose sole prior loss came at super middleweight against John Ryder in 2022 after damaging his hand in the contest, was making just his second appearance at the higher weight class.
By the halfway stage, the underdog had made it competitive, creating difficulties for the favorite, who utilized a solid defense to narrowly win the cleaner rounds.
The crowd's patience diminished amid frequent holding, with Parker going down multiple occasions during grappling.
The underdog finished the stronger, but several stanzas were difficult to score. Although, previous titleholder Carl Frampton - working with the broadcast network - had it significantly in favor for Parker, giving Buatsi just a single round.
Buatsi moves on, but with the likes of hard hitter David Benavidez, and Russian champion Bivol leading the category, the road to world glory - on this evidence - still seems difficult.
Ricky Hatton Tribute Inspires All-Manchester Contest
In the co-main event, Arthur overcame Rea by split-decision in a outstanding all-Manchester contest to capture the European light-heavyweight belt.
The arena became quiet for a deeply poignant homage ahead of the contest as 10 bells sounded in memory of the city's native Hatton, who succumbed aged 46 in recently.
It was the premier televised boxing event in the city since the passing of Hatton. As the final chime ended, the silence became a chant of "there's only one Ricky Hatton".
The competition within the ring equaled the feeling. The eventual winner was floored by a powerful punch directly to the chin in the initial stage, getting up unsteadily to his footing on the count of eight.
But the traditional red equipment of Arthur soon advanced again. From the other corner, the opponent - in opposing blue - was marked up quickly, heavy swelling forming under his face as the pair exchanged punishing blows.
Rea, 27, connected with strong blows but the champion, 34, responded in kind with stinging uppercuts as the intensity rarely dropped over twelve rounds.
The victor edged the result with judges scorecards of draw, close margin and clearer margin, but each competitor were persistent and courageous, everything their inspiration Hatton stood for.
