British light-heavyweight Buatsi earned a win yet underwhelming start to his career under new promoters with a decision win over Parker in Manchester.
The Olympic medal winner's punch output likely gained him a majority decision in a tight ten-round contest at the Co-op Live Arena.
But Parker landed significant shots, delivering effective individual punches in a fight where the styles failed to mesh, with frequent clinching marring the action.
The officials judged it 96-94 for to the winner, with the third a 95-95 draw.
"It wasn't my finest performance but what defines champions? They win," the victor commented in the ring as he was met by disapproval from the spectators.
The Londoner returned to winning ways after his initial pro loss - an epic battle with his previous opponent in February - to claim a twentieth professional win.
Several boxing experts believed the challenger, 31, merited the win but the contender endured the second loss of his 28-match professional journey.
Buatsi Secures Victory in Dull Affair
His career development has slowed in the past few years through contract problems and long spells of inactivity.
Earlier forecast as the next star of UK boxing, he now is positioned trying to achieve expectations - perhaps in what seems like a ultimate opportunity - under recent support.
He started carefully, with both competitors ready to be aggressive in a calculated initial rounds.
Parker, whose sole prior loss came at super-middleweight against John Ryder in 2022 after suffering a hand injury during the bout, was competing in only his second fight at light heavyweight.
By the middle rounds, Parker had kept it close, causing problems for Buatsi, who operated with a protective stance to edge the cleaner stanzas.
The crowd's patience diminished amid constant clinching, with the challenger falling to the floor several times during grappling.
The underdog closed well, but several stanzas were difficult to score. Nevertheless, former world champion Carl Frampton - commentating on the broadcast network - had it strongly leaning for Parker, giving Buatsi just one round.
The victor moves on, but with the presence of hard hitter David Benavidez, and Eastern European fighter Bivol ruling the division, the road to world glory - on this evidence - still appears challenging.
Ricky Hatton Tribute Fuels Local Rivalry Battle
In the co-main event, Lyndon Arthur defeated Bradley Rea by divided decision in a terrific Manchester vs Manchester fight to win the European light-heavyweight belt.
The arena grew hushed for a deeply poignant memorial before the fight as 10 bells sounded in remembrance of the city's native Ricky Hatton, who died aged 46 in September.
It was the first major televised fight night in the location since Hatton's death. As the concluding sound disappeared, the stillness transitioned to a shout of "there's just one Ricky Hatton".
The competition within the ring matched the feeling. The eventual winner was sent to the canvas by a significant blow directly to the jaw in the initial stage, getting up unsteadily to his footing on the referee's count.
But the red-colored equipment of Arthur soon advanced again. Across the ring, Rea - in opposing blue - was damaged early, noticeable damage forming under his orbital area as the duo landed powerful shots.
The challenger, 27, connected with strong blows but Arthur, thirty-four, fought fire with fire with stinging uppercuts as the intensity hardly diminished over twelve rounds.
Arthur narrowly won the verdict with official scores of 114-114, close margin and 115-112, but each competitor were persistent and full of heart, everything their inspiration Hatton represented.
