UK light-heavyweight Buatsi earned a win yet unimpressive start to his career under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's punch output appeared to earn him a majority decision in a tight 10-round bout at the Manchester venue.
But the challenger had his moments, connecting with powerful single shots in a match where the techniques didn't complement each other, with excessive holding marring the action.
Two judges judged it 96-94 for to the winner, with the remaining official a even score.
"It wasn't my best display but how do champions respond? They win," the victor said in the ringside as he was greeted with disapproval from the crowd.
The British fighter bounced back after his initial pro loss - an memorable fight with his previous opponent in February - to claim a 20th pro success.
Various analysts believed the challenger, 31, deserved the victory but the contender experienced the second loss of his 28-bout professional journey.
Buatsi Claims Victory in Uneventful Match
The fighter's advancement has slowed in recent years through promotional disputes and extended periods of inactivity.
Once tipped as the rising talent of UK boxing, he now finds himself trying to realize his potential - possibly in what appears to be a ultimate opportunity - under new promotional backing.
He started carefully, with neither fighter ready to overcommit in a tentative early stages.
Parker, whose sole prior loss came at super-middleweight against his former opponent in two years ago after damaging his hand mid-fight, was participating in his second bout at light heavyweight.
By the halfway stage, Parker had stayed in contention, creating difficulties for Buatsi, who operated with a protective stance to barely take the cleaner stanzas.
The audience's tolerance diminished during regular grabbing, with the challenger falling to the floor several times during clinches.
The underdog finished the stronger, but multiple periods were challenging to assess. However, previous titleholder Carl Frampton - commentating on the broadcast network - had it particularly wide for Parker, scoring for Buatsi just only one stanza.
Buatsi progresses, but with the presence of heavy hitter Benavidez, and Russian star Bivol leading the category, the journey to titles - based on this performance - still looks steep.
Hatton Homage Motivates Manchester Derby Battle
In the supporting bout, Lyndon Arthur overcame Bradley Rea by split-decision in a terrific Manchester vs Manchester fight to win the European light heavyweight championship.
The arena grew hushed for a profoundly moving homage prior to the bout as a bell ceremony tolled in memory of Manchester's own Ricky Hatton, who succumbed at age 46 in September.
It was the premier televised boxing event in the urban center since Hatton's passing. As the final chime ended, the stillness transitioned to a roar of "there's just one Hatton".
The competition within the ring equaled the sentiment. Arthur was floored by a significant blow flush on the face in the early round, getting up unsteadily to his position on the count of eight.
But the traditional red shorts of Arthur soon advanced again. On the opposite side, the opponent - in competitive blue - was damaged early, noticeable damage developing under his orbital area as the fighters landed hard punches.
The challenger, twenty-seven, connected with strong blows but the champion, 34, responded in kind with powerful upward punches as the pace hardly diminished over 12 rounds.
The victor barely secured the verdict with official scores of 114-114, close margin and clearer margin, but the two athletes were relentless and brave, all the qualities their inspiration Hatton stood for.
