UK light-heavyweight Buatsi made a winning yet underwhelming start to his career under new promoters with a unanimous decision over Parker in Manchester.
The Olympic medal winner's activity level seemingly secured him a split decision in a tight ten-round contest at the Manchester venue.
But the challenger landed significant shots, landing some telling clean blows in a fight where the approaches failed to mesh, with excessive holding disrupting the fight.
Two judges had it 96-94 for to the winner, with the final judge a 95-95.
"It wasn't my best performance but how do champions respond? They succeed," the victor stated in the arena as he was met by negative reactions from the spectators.
The British fighter returned to winning ways after his first professional defeat - an thrilling contest with his previous opponent in last winter - to secure a 20th pro success.
Several boxing experts thought Parker, 31, merited the decision but the British boxer suffered the second loss of his 28-match professional journey.
The Champion Earns Triumph in Dull Affair
Buatsi's progress has stalled in recent times through contract problems and extended periods of inactivity.
Previously predicted as the rising talent of British boxing, he now finds himself trying to achieve expectations - perhaps in what feels like a ultimate opportunity - under recent support.
He opened tentatively, with neither fighter ready to be aggressive in a tentative early stages.
The challenger, whose only previous defeat came at the lower weight class against John Ryder in 2022 after breaking his hand during the bout, was participating in his second fight at the higher weight class.
By the fight's midpoint, the underdog had stayed in contention, making life awkward for the favorite, who operated with a protective stance to edge the better stanzas.
The spectators' acceptance diminished amid constant clinching, with the challenger going down various instances during grappling.
The underdog closed well, but many rounds were difficult to score. Although, former world champion Carl Frampton - working with the broadcast network - had it particularly wide for the challenger, awarding Buatsi just only one stanza.
Buatsi advances, but with the caliber of hard hitter Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the path to championship success - on this evidence - still looks steep.
Hatton Tribute Inspires Manchester Derby Contest
In the co-main event, Lyndon Arthur defeated Bradley Rea by close verdict in a outstanding all-Manchester fight to capture the European light heavyweight championship.
The venue became quiet for a extremely emotional memorial ahead of the contest as 10 bells rang in honor of Manchester's own Ricky Hatton, who succumbed aged 46 in last month.
It was the first major televised fight night in the urban center since Hatton's passing. As the final chime faded, the quiet gave way to a shout of "there's just one Hatton".
The fighting inside the ropes matched the emotion. Arthur was knocked down by a powerful punch flush on the face in the early round, getting up unsteadily to his footing on the count of eight.
But the Manchester United-red shorts of the fighter soon came forward again. From the other corner, the opponent - in opposing blue - was injured quickly, noticeable damage forming under his orbital area as the duo traded powerful shots.
Rea, twenty-seven, landed power punches but Arthur, thirty-four, matched the intensity with stinging uppercuts as the pace rarely dropped over 12 rounds.
The victor edged the result with scoring tallies of 114-114, close margin and wider gap, but each competitor were relentless and courageous, exactly what their hero the legend represented.
