British light heavyweight Buatsi earned a win yet disappointing start to his career under new promoters with a points victory over Parker in Manchester.
The Olympic medal winner's punch output appeared to earn him a majority decision in a close 10-round contest at the Manchester venue.
But Parker landed significant shots, delivering effective single shots in a contest where the approaches failed to mesh, with excessive holding disrupting the fight.
The officials had it 96-94 to Buatsi, with the final judge a 95-95 draw.
"It wasn't my greatest display but how do champions respond? They win," Buatsi commented in the ringside as he was greeted with disapproval from the audience.
The British fighter returned to winning ways after his initial pro loss - an thrilling contest with Callum Smith in last winter - to secure a twentieth professional win.
Various analysts believed Parker, thirty-one, deserved the victory but the contender endured the second loss of his 28-fight career.
The Champion Secures Win in Slow Affair
His career development has plateaued in recent times through contract problems and long spells of inactivity.
Earlier forecast as the future of the British fight scene, he now finds himself trying to realize his potential - possibly in what feels like a ultimate opportunity - under fresh management.
He started carefully, with neither fighter ready to be aggressive in a tentative opening few rounds.
The challenger, whose single earlier setback came at the lower weight class against John Ryder in 2022 after suffering a hand injury during the bout, was participating in his second bout at light-heavyweight.
By the middle rounds, the underdog had made it competitive, causing problems for the favorite, who operated with a solid defense to narrowly win the more effective periods.
The crowd's patience wore thin amid frequent holding, with the challenger slipping to the canvas several times during close exchanges.
Parker closed well, but many rounds were difficult to score. However, ex-champion the boxing analyst - working with Dazn - had it significantly in favor for the challenger, scoring for Buatsi just one round.
The victor moves on, but with the caliber of hard hitter Benavidez, and Russian star Dmitry Bivol leading the category, the journey to titles - based on this performance - still looks steep.
Hatton Memorial Motivates Manchester Derby Battle
In the supporting bout, Arthur defeated Bradley Rea by divided decision in a outstanding local rivalry contest to win the European light heavyweight title.
The arena became quiet for a deeply poignant memorial prior to the bout as ten bells rang in honor of Manchester's own Hatton, who died 46 years old in recently.
It was the initial significant broadcast match card in the city since the passing of Hatton. As the last bell faded, the quiet became a shout of "there's just one Hatton".
The action inside the ropes matched the emotion. The eventual winner was sent to the canvas by a left hook flush on the face in the initial stage, standing uncertainly to his feet on the eight count.
But the traditional red equipment of Arthur soon came forward again. Across the ring, Rea - in opposing blue - was damaged soon, heavy swelling forming under his orbital area as the duo exchanged powerful shots.
The challenger, twenty-seven, connected with strong blows but the champion, thirty-four, responded in kind with stinging uppercuts as the intensity rarely dropped over 12 rounds.
Arthur barely secured the result with scoring tallies of draw, narrow lead and wider gap, but the two athletes were determined and courageous, exactly what their hero Hatton represented.
