British light-heavyweight Buatsi secured a victory yet disappointing debut to his career under recent management with a points victory over Parker in Manchester.
The Olympic medal winner's work rate likely gained him a split decision in a tight ten-round contest at the Co-op Live Arena.
But Parker landed significant shots, delivering effective clean blows in a match where the techniques never quite clicked, with frequent clinching spoiling the action.
Two judges had it 96-94 in favor to Buatsi, with the remaining official a 95-95.
"It wasn't my best performance but how do champions respond? They succeed," the victor commented in the ringside as he was met by boos from the audience.
The Londoner returned to winning ways after his initial pro loss - an thrilling contest with his previous opponent in February - to secure a 20th career victory.
Several boxing experts thought the challenger, thirty-one, should have been awarded the win but the British boxer endured the second defeat of his 28-bout professional journey.
Buatsi Claims Triumph in Slow Affair
His career development has slowed in recent times through promotional disputes and long spells of inactivity.
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 recent support.
He started carefully, with the two boxers ready to take risks in a calculated opening few rounds.
Parker, whose only previous defeat came at the lower weight class against John Ryder in 2022 after breaking his hand during the bout, was competing in only his second appearance at light-heavyweight.
By the fight's midpoint, Parker had stayed in contention, making life awkward for the favorite, who worked behind a tight guard to narrowly win the better stanzas.
The crowd's patience diminished during frequent holding, with Parker falling to the floor multiple occasions during close exchanges.
Parker closed well, but several stanzas were challenging to assess. Nevertheless, former world champion the boxing analyst - commentating on Dazn - had it strongly leaning for the challenger, scoring for Buatsi just one round.
The victor advances, but with the caliber of hard hitter David Benavidez, and Russian star Dmitry Bivol dominating the weight class, the path to championship success - based on this performance - still appears challenging.
Hatton Tribute Inspires Local Rivalry Contest
In the secondary fight, Lyndon Arthur beat Rea by divided decision in a outstanding local rivalry fight to secure the European light heavyweight belt.
The venue fell silent for a profoundly moving tribute prior to the bout as ten bells sounded in honor of the city's native Hatton, who died at age 46 in recently.
It was the premier televised match card in the urban center since the passing of Hatton. As the final chime ended, the silence gave way to a chant of "only one Hatton".
The action during the bout reflected the sentiment. The eventual winner was sent to the canvas by a powerful punch directly to the face in the initial stage, getting up unsteadily to his feet on the eight count.
But the red-colored equipment of Arthur soon came forward again. On the opposite side, the opponent - in opposing blue - was marked up quickly, noticeable damage developing under his face as the fighters exchanged hard punches.
Rea, twenty-seven, delivered effective strikes but the champion, thirty-four, matched the intensity with powerful upward punches as the pace rarely dropped over twelve rounds.
The victor narrowly won the result with scoring tallies of even score, narrow lead and 115-112, but each competitor were persistent and courageous, all the qualities their inspiration the legend stood for.
