British light heavyweight Buatsi earned a win yet unimpressive debut to his career under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic medal winner's work rate seemingly secured him a majority decision in a competitive ten-round bout at the boxing arena.
But the challenger found success, connecting with powerful clean blows in a fight where the styles failed to mesh, with frequent clinching marring the contest.
The officials had it 96-94 in favor to the winner, with the final judge a even score.
"It wasn't my best performance but how do champions respond? They win," the victor commented in the ringside as he was greeted with disapproval from the audience.
The Londoner bounced back after his initial pro loss - an memorable fight with Callum Smith in last winter - to earn a twentieth professional win.
Various analysts felt Parker, 31, should have been awarded the win but the Derbyshire fighter suffered the second setback of his 28-fight career.
Buatsi Earns Victory in Dull Encounter
Buatsi's progress has stalled in recent times through contract problems and extended periods of lack of fights.
Earlier forecast as the future of the British fight scene, he now is positioned trying to realize his potential - possibly in what appears to be a final roll of the dice - under recent support.
He began cautiously, with neither fighter willing to be aggressive in a cagey opening few rounds.
The challenger, whose single earlier setback came at super-middleweight against John Ryder in 2022 after suffering a hand injury in the contest, was making just his second fight at light heavyweight.
By the middle rounds, Parker had made it competitive, creating difficulties for the favorite, who worked behind a protective stance to edge the more effective rounds.
The spectators' acceptance diminished because of frequent holding, with Parker going down multiple occasions during grappling.
The underdog ended stronger, but several stanzas were challenging to assess. However, ex-champion Carl Frampton - providing analysis for the broadcast network - had it strongly leaning for the challenger, awarding Buatsi just a single round.
The victor progresses, but with the likes of heavy hitter David Benavidez, and Russian star Bivol dominating the weight class, the path to championship success - based on this performance - still appears challenging.
Ricky Hatton Tribute Fuels Local Rivalry Fight
In the co-main event, Lyndon Arthur defeated Rea by close verdict in a outstanding Manchester vs Manchester battle to capture the European light heavyweight title.
The venue became quiet for a deeply poignant memorial before the fight as 10 bells rang in remembrance of Manchester's own Ricky Hatton, who died at age 46 in September.
It was the initial significant broadcast boxing event in the urban center since Hatton's passing. As the last bell disappeared, the silence transitioned to a chant of "there's only one Hatton".
The fighting during the bout reflected the sentiment. The eventual winner was floored by a powerful punch squarely on the face in the initial stage, rising shakily to his position on the count of eight.
But the red-colored shorts of Arthur soon pressed ahead again. Across the ring, the opponent - in opposing blue - was injured quickly, heavy swelling forming under his face as the duo exchanged hard punches.
Rea, 27, connected with strong blows but the champion, 34, fought fire with fire with powerful upward punches as the intensity hardly diminished over 12 rounds.
Arthur narrowly won the decision with scoring tallies of even score, 115-113 and 115-112, but each competitor were relentless and full of heart, exactly what their hero the legend stood for.
