British light-heavyweight Buatsi earned a win yet unimpressive debut to his career under new promoters with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output likely gained him a majority decision in a competitive 10-round contest at the Co-op Live Arena.
But Parker found success, landing some telling clean blows in a fight where the approaches didn't complement each other, with frequent clinching spoiling the action.
The scoring judges had it 96-94 in favor to Buatsi, with the final judge a even score.
"It wasn't my greatest display but how do champions respond? They win," the victor commented in the ring as he was greeted with boos from the spectators.
The Londoner bounced back after his initial pro loss - an epic battle with his previous opponent in last winter - to secure a 20th pro success.
Several boxing experts felt Parker, thirty-one, merited the victory but the Derbyshire fighter experienced the second loss of his 28-match professional journey.
The Champion Claims Triumph in Uneventful Match
The fighter's advancement has slowed in recent years through contract problems and considerable time of lack of fights.
Previously predicted as the next star of UK boxing, he now stands trying to achieve expectations - perhaps in what feels like a final roll of the dice - under recent support.
He began cautiously, with both competitors ready to be aggressive in a tentative early stages.
Parker, whose only previous defeat came at the lower weight class against John Ryder in 2022 after suffering a hand injury in the contest, was competing in only his second fight at the higher weight class.
By the fight's midpoint, Parker had stayed in contention, creating difficulties for the favorite, who operated with a tight guard to barely take the cleaner periods.
The spectators' acceptance diminished amid frequent holding, with Parker going down several times during grappling.
Parker ended stronger, but many rounds were challenging to assess. However, ex-champion Carl Frampton - working with the broadcast network - had it particularly wide for the challenger, awarding Buatsi just one round.
Buatsi progresses, but with the caliber of hard hitter David Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the road to world glory - based on this performance - still looks steep.
Hatton Tribute Motivates Manchester Derby Fight
In the supporting bout, Lyndon Arthur defeated Bradley Rea by split-decision in a excellent Manchester vs Manchester contest to win the European light-heavyweight title.
The arena became quiet for a profoundly moving memorial ahead of the contest as ten bells sounded in memory of the city's native Hatton, who succumbed at age 46 in September.
It was the first major televised boxing event in the urban center since Hatton's passing. As the final chime disappeared, the quiet became a chant of "there's only one Hatton".
The competition inside the ropes equaled the feeling. The eventual winner was knocked down by a significant blow directly to the chin in the second round, standing uncertainly to his feet on the eight count.
But the traditional red equipment of the fighter soon came forward again. On the opposite side, the opponent - in rival City blue - was damaged quickly, heavy swelling appearing under his orbital area as the pair exchanged punishing blows.
The challenger, twenty-seven, delivered effective strikes but Arthur, thirty-four, matched the intensity with stinging uppercuts as the pace seldom decreased over the full distance.
The victor narrowly won the result with official scores of 114-114, 115-113 and wider gap, but both fighters were relentless and full of heart, everything their idol Hatton stood for.
