British light heavyweight Buatsi made a winning yet unimpressive start to his career under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's work rate seemingly secured him a split decision in a close 10-round contest at the Co-op Live Arena.
But Parker found success, delivering effective single shots in a contest where the styles didn't complement each other, with constant grabbing spoiling the fight.
Two judges scored it 96-94 in favor to Buatsi, with the final judge a even score.
"It wasn't my finest display but what defines champions? They win," Buatsi commented in the ring as he was greeted with boos from the spectators.
The British fighter bounced back after his maiden career setback - an epic battle with Callum Smith in February - to earn a 20th career victory.
Some ringside observers believed the challenger, 31, should have been awarded the victory but the contender experienced the second defeat of his 28-match career.
Buatsi Earns Triumph in Slow Affair
Buatsi's progress has slowed in recent years through promotional disputes and considerable time of inactivity.
Earlier forecast as the future of British boxing, he now finds himself trying to fulfil that early promise - perhaps in what seems like a final roll of the dice - under new promotional backing.
He opened tentatively, with both competitors prepared to be aggressive in a calculated early stages.
Parker, whose sole prior loss came at super middleweight against John Ryder in 2022 after suffering a hand injury during the bout, was participating in his second appearance at light heavyweight.
By the middle rounds, Parker had made it competitive, making life awkward for the favorite, who worked behind a tight guard to edge the more effective rounds.
The spectators' acceptance wore thin because of frequent holding, with Parker falling to the floor several times during grappling.
The underdog ended stronger, but multiple periods were hard to judge. However, ex-champion Carl Frampton - commentating on the broadcast network - had it significantly in favor for the challenger, giving Buatsi just one round.
The victor moves on, but with the likes of power puncher David Benavidez, and Russian star Bivol leading the category, the road to world glory - based on this performance - still looks steep.
Ricky Hatton Tribute Motivates Local Rivalry Contest
In the co-main event, Lyndon Arthur beat Rea by split-decision in a terrific local rivalry fight to capture the European light-heavyweight championship.
The venue grew hushed for a profoundly moving tribute prior to the bout as 10 bells tolled in remembrance of Manchester's own Ricky Hatton, who died aged 46 in recently.
It was the premier televised boxing event in the urban center since the passing of Hatton. As the last bell disappeared, the stillness transitioned to a roar of "only one Hatton".
The action during the bout reflected the feeling. The eventual winner was sent to the canvas by a left hook flush on the face in the second round, rising shakily to his footing on the eight count.
But the Manchester United-red equipment of Arthur soon came forward again. On the opposite side, Rea - in opposing blue - was damaged quickly, significant bruising forming under his eye as the pair traded punishing blows.
The challenger, 27, delivered effective strikes but the champion, thirty-four, responded in kind with powerful upward punches as the pace seldom decreased over the full distance.
Arthur narrowly won the verdict with judges scorecards of draw, narrow lead and 115-112, but the two athletes were relentless and brave, exactly what their idol the legend embodied.
