UK light-heavyweight Buatsi earned a win yet unimpressive start to his career under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's punch output likely gained him a judges' verdict in a tight ten-round bout at the Co-op Live Arena.
But the challenger had his moments, landing some telling individual punches in a fight where the styles failed to mesh, with excessive holding marring the fight.
The scoring judges scored it 96-94 for to the winner, with the final judge a 95-95 draw.
"It wasn't my greatest showing but what do champions do? They win," the victor stated in the ring as he was greeted with negative reactions from the spectators.
The British fighter got back to winning after his initial pro loss - an epic battle with Callum Smith in last winter - to secure a twentieth professional win.
Some ringside observers believed Parker, 31, deserved the win but the contender endured the second setback of his 28-match professional journey.
Buatsi Secures Triumph in Slow Encounter
His career development has plateaued in recent times through promotional disputes and extended periods of lack of fights.
Previously predicted as the next star of the British fight scene, he now stands trying to fulfil that early promise - perhaps in what appears to be a final roll of the dice - under fresh management.
He began cautiously, with neither fighter ready to overcommit in a tentative initial rounds.
The challenger, whose single earlier setback came at the lower weight class against his former opponent in two years ago after damaging his hand in the contest, was competing in only his second fight at light heavyweight.
By the halfway stage, Parker had made it competitive, causing problems for Buatsi, who worked behind a solid defense to edge the cleaner rounds.
The audience's tolerance wore thin during frequent holding, with Parker going down various instances during close exchanges.
Parker finished the stronger, but many rounds were difficult to score. Nevertheless, previous titleholder the boxing analyst - commentating on the broadcast network - had it particularly wide for Parker, awarding Buatsi just one round.
The victor moves on, but with the likes of heavy hitter David Benavidez, and Russian star Bivol ruling the division, the road to world glory - judging by this showing - still appears challenging.
Hatton Memorial Fuels Local Rivalry Fight
In the co-main event, Lyndon Arthur beat Bradley Rea by close verdict in a terrific all-Manchester contest to capture the continental title.
The venue became quiet for a extremely emotional homage before the fight as a bell ceremony sounded in honor of the city's native Hatton, who succumbed at age 46 in recently.
It was the initial significant broadcast boxing event in the location since Hatton's passing. As the concluding sound disappeared, the quiet transitioned to a shout of "there's just one Hatton".
The competition inside the ropes reflected the sentiment. Arthur was floored by a left hook flush on the chin in the second round, rising shakily to his position on the referee's count.
But the red-colored equipment of the fighter soon pressed ahead again. From the other corner, Rea - in competitive blue - was injured quickly, heavy swelling appearing under his orbital area as the pair landed powerful shots.
Rea, twenty-seven, landed power punches but Arthur, thirty-four, responded in kind with powerful upward punches as the action seldom decreased over the full distance.
The victor narrowly won the decision with judges scorecards of 114-114, narrow lead and clearer margin, but both fighters were persistent and full of heart, all the qualities their hero the legend stood for.
