British light-heavyweight Buatsi earned a win yet unimpressive beginning to life under new promoters with a decision win over Parker in Manchester.
The Olympic medal winner's work rate appeared to earn him a split decision in a close ten-round fight at the Manchester venue.
But Parker had his moments, connecting with powerful individual punches in a fight where the approaches didn't complement each other, with excessive holding disrupting the fight.
The scoring judges scored it 96-94 for to Buatsi, with the remaining official a even score.
"It wasn't my best showing but what do champions do? They succeed," the victor said in the ring as he was faced disapproval from the spectators.
The British fighter bounced back after his maiden career setback - an thrilling contest with Callum Smith in last winter - to claim a twentieth professional win.
Some ringside observers felt the challenger, thirty-one, merited the win but the British boxer endured the second defeat of his 28-match career.
Buatsi Secures Victory in Slow Encounter
Buatsi's progress has plateaued in recent years through management issues and extended periods of lack of fights.
Previously predicted as the rising talent of the British fight scene, he now is positioned trying to fulfil that early promise - maybe in what feels like a last chance - under fresh management.
He opened tentatively, with both competitors ready to overcommit in a cagey initial rounds.
The challenger, whose single earlier setback came at the lower weight class against his former opponent in 2022 after suffering a hand injury during the bout, was making just his second fight at light-heavyweight.
By the middle rounds, the underdog had kept it close, making life awkward for the favorite, who operated with a solid defense to barely take the cleaner periods.
The spectators' acceptance wore thin during constant clinching, with the challenger falling to the floor multiple occasions during clinches.
Parker closed well, but several stanzas were hard to judge. However, previous titleholder the boxing analyst - working with Dazn - had it particularly wide for Parker, awarding Buatsi just only one stanza.
Buatsi moves on, but with the presence of hard hitter David Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the road to world glory - based on this performance - still appears challenging.
Hatton Memorial Inspires Manchester Derby Contest
In the supporting bout, Arthur defeated Rea by split-decision in a terrific all-Manchester contest to secure the European light heavyweight title.
The stadium grew hushed for a deeply poignant homage before the fight as ten bells rang in remembrance of the city's native Ricky Hatton, who passed away at age 46 in last month.
It was the premier televised boxing event in the urban center since the passing of Hatton. As the last bell faded, the silence became a shout of "there's only one Ricky Hatton".
The competition inside the ropes reflected the sentiment. The eventual winner was sent to the canvas by a left hook directly to the chin in the initial stage, rising shakily to his position on the eight count.
But the red-colored shorts of Arthur soon pressed ahead again. On the opposite side, Rea - in opposing blue - was damaged quickly, heavy swelling developing under his orbital area as the fighters traded hard punches.
The challenger, 27, landed power punches but the champion, 34, matched the intensity with stinging uppercuts as the action hardly diminished over the full distance.
The victor narrowly won the verdict with official scores of even score, narrow lead and clearer margin, but the two athletes were persistent and courageous, all the qualities their hero Hatton stood for.
