British light heavyweight Buatsi earned a win yet underwhelming beginning to life under new promoters with a decision win over Parker in Manchester.
The Olympic bronze medallist's punch output seemingly secured him a split decision in a close 10-round contest at the Co-op Live Arena.
But Parker landed significant shots, connecting with powerful clean blows in a contest where the techniques never quite clicked, with excessive holding disrupting the action.
Two judges scored it 96-94 in favor to the winner, with the final judge a even score.
"It wasn't my best display but what do champions do? They win," the victor said in the arena as he was met by disapproval from the audience.
The Londoner bounced back after his maiden career setback - an thrilling contest with his previous opponent in last winter - to claim a twentieth professional win.
Various analysts believed the challenger, 31, merited the win but the Derbyshire fighter experienced the second setback of his 28-bout career.
Buatsi Claims Victory in Slow Affair
His career development has stalled in the past few years through management issues and considerable time of lack of fights.
Once tipped as the rising talent of UK boxing, he now finds himself trying to fulfil that early promise - possibly in what appears to be a last chance - under recent support.
He started carefully, with neither fighter prepared to overcommit in a calculated early stages.
Parker, whose only previous defeat came at the lower weight class against his former opponent in two years ago after suffering a hand injury during the bout, was participating in his second fight at light-heavyweight.
By the middle rounds, Parker had made it competitive, causing problems for the favorite, who worked behind a protective stance to barely take the better stanzas.
The audience's tolerance diminished because of frequent holding, with Parker going down several times during grappling.
Parker finished the stronger, but several stanzas were challenging to assess. Nevertheless, previous titleholder the boxing analyst - commentating on Dazn - had it strongly leaning for Parker, awarding Buatsi just one round.
Buatsi moves on, but with the presence of hard hitter Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the journey to titles - judging by this showing - still looks steep.
Ricky Hatton Homage Inspires All-Manchester Contest
In the secondary fight, Lyndon Arthur overcame Bradley Rea by divided decision in a terrific local rivalry fight to secure the European light-heavyweight title.
The venue became quiet for a extremely emotional memorial ahead of the contest as ten bells tolled in honor of the city's native Ricky Hatton, who died 46 years old in recently.
It was the initial significant broadcast boxing event in the urban center since Hatton's passing. As the last bell faded, the silence gave way to a shout of "there's only one Hatton".
The action within the ring reflected the sentiment. The eventual winner was floored by a powerful punch directly to the jaw in the second round, standing uncertainly to his position on the count of eight.
But the red-colored equipment of the fighter soon pressed ahead again. Across the ring, Rea - in rival City blue - was injured early, heavy swelling appearing under his eye as the fighters exchanged powerful shots.
Rea, 27, connected with strong blows but Arthur, thirty-four, matched the intensity with stinging uppercuts as the pace hardly diminished over twelve rounds.
Arthur narrowly won the verdict with official scores of 114-114, close margin and wider gap, but the two athletes were persistent and brave, exactly what their idol the legend stood for.
