British light-heavyweight Buatsi made a winning yet unimpressive beginning to his career under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic bronze medallist's work rate likely gained him a split decision in a close 10-round fight at the Co-op Live Arena.
But the challenger had his moments, landing some telling single shots in a contest where the approaches didn't complement each other, with constant grabbing spoiling the contest.
Two judges had it 96-94 for to Buatsi, with the final judge a even score.
"It wasn't my best showing but what defines champions? They win," Buatsi commented in the ring as he was faced boos from the audience.
The Londoner returned to winning ways after his maiden career setback - an memorable fight with Callum Smith in last winter - to secure a 20th pro success.
Several boxing experts believed the challenger, 31, merited the decision but the Derbyshire fighter experienced the second setback of his 28-match professional journey.
Buatsi Claims Triumph in Slow Affair
His career development has slowed in recent times through management issues and long spells of inactivity.
Earlier forecast as the rising talent of UK boxing, he now finds himself trying to achieve expectations - possibly in what seems like a last chance - under recent support.
He began cautiously, with both competitors prepared to take risks in a tentative initial rounds.
The challenger, whose only previous defeat came at the lower weight class against John Ryder in 2022 after suffering a hand injury mid-fight, was making just his second fight at light-heavyweight.
By the middle rounds, Parker had stayed in contention, making life awkward for the favorite, who worked behind a solid defense to edge the more effective stanzas.
The crowd's patience wore thin amid constant clinching, with Parker falling to the floor various instances during clinches.
The underdog finished the stronger, but several stanzas were challenging to assess. Although, previous titleholder the boxing analyst - working with Dazn - had it strongly leaning for Parker, giving Buatsi just one round.
Buatsi advances, but with the likes of hard hitter David Benavidez, and Russian champion Bivol ruling the division, the road to world glory - judging by this showing - still seems difficult.
Hatton Memorial Motivates All-Manchester Battle
In the supporting bout, Lyndon Arthur overcame Rea by divided decision in a excellent all-Manchester battle to win the European light heavyweight championship.
The arena grew hushed for a deeply poignant homage ahead of the contest as 10 bells rang in remembrance of the city's native Ricky Hatton, who passed away at age 46 in last month.
It was the initial significant broadcast fight night in the urban center since the passing of Hatton. As the concluding sound faded, the stillness gave way to a roar of "only one Ricky Hatton".
The fighting inside the ropes matched the emotion. The eventual winner was sent to the canvas by a left hook directly to the face in the initial stage, standing uncertainly to his position on the referee's count.
But the red-colored equipment of Arthur soon advanced again. From the other corner, Rea - in opposing blue - was injured quickly, heavy swelling forming under his eye as the duo landed punishing blows.
Rea, 27, delivered effective strikes but the champion, 34, fought fire with fire with stinging uppercuts as the action rarely dropped over the full distance.
The victor narrowly won the decision with judges scorecards of 114-114, narrow lead and clearer margin, but the two athletes were relentless and brave, exactly what their hero the legend stood for.
