British light heavyweight Buatsi secured a victory yet unimpressive beginning to his career under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic medal winner's work rate seemingly secured him a split decision in a competitive ten-round fight at the Co-op Live Arena.
But the challenger found success, landing some telling clean blows in a contest where the approaches didn't complement each other, with frequent clinching spoiling the action.
The scoring judges had it 96-94 to the winner, with the third a even score.
"It wasn't my finest display but how do champions respond? They win," Buatsi stated in the ring as he was greeted with boos from the crowd.
The Londoner returned to winning ways after his first professional defeat - an memorable fight with Callum Smith in last winter - to earn a 20th career victory.
Some ringside observers thought the challenger, thirty-one, merited the decision but the British boxer experienced the second setback of his 28-fight career.
Buatsi Claims Victory in Uneventful Affair
His career development has stalled in the past few years through management issues and extended periods of lack of fights.
Previously predicted as the future of UK boxing, he now is positioned trying to fulfil that early promise - possibly in what feels like a last chance - under fresh management.
He started carefully, with both competitors willing to be aggressive in a tentative early stages.
Parker, whose only previous defeat came at the lower weight class against his former opponent in 2022 after suffering a hand injury mid-fight, was participating in his second bout at light heavyweight.
By the halfway stage, the underdog had made it competitive, creating difficulties for the favorite, who utilized a protective stance to edge the cleaner rounds.
The spectators' acceptance decreased amid frequent holding, with Parker falling to the floor several times during close exchanges.
Parker finished the stronger, but many rounds were hard to judge. Nevertheless, previous titleholder the boxing analyst - working with the broadcast network - had it significantly in favor for the challenger, giving Buatsi just one round.
Buatsi moves on, but with the likes of hard hitter David Benavidez, and Russian star Dmitry Bivol leading the category, the path to championship success - on this evidence - still seems difficult.
Ricky Hatton Homage Inspires Manchester Derby Fight
In the supporting bout, Lyndon Arthur beat Bradley Rea by split-decision in a outstanding local rivalry battle to secure the continental championship.
The stadium grew hushed for a extremely emotional homage ahead of the contest as ten bells tolled in memory of local hero Hatton, who succumbed aged 46 in September.
It was the first major televised match card in the urban center since the passing of Hatton. As the final chime disappeared, the stillness became a roar of "there's only one Ricky Hatton".
The action inside the ropes matched the feeling. The eventual winner was floored by a left hook squarely on the jaw in the second round, standing uncertainly to his position on the eight count.
But the Manchester United-red shorts of Arthur soon came forward again. Across the ring, the opponent - in rival City blue - was injured soon, heavy swelling forming under his face as the pair traded powerful shots.
Rea, twenty-seven, landed power punches but the champion, 34, responded in kind with stinging uppercuts as the intensity hardly diminished over the full distance.
The victor edged the result with scoring tallies of draw, close margin and clearer margin, but each competitor were persistent and full of heart, all the qualities their idol Hatton represented.
