British light-heavyweight Buatsi secured a victory yet unimpressive beginning to his career under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's activity level likely gained him a split decision in a tight 10-round contest at the Co-op Live Arena.
But the challenger landed significant shots, delivering effective clean blows in a contest where the approaches never quite clicked, with frequent clinching spoiling the fight.
The officials scored it 96-94 in favor to the winner, with the remaining official a even score.
"It wasn't my finest display but what do champions do? They win," the victor commented in the ring as he was greeted with boos from the audience.
The Londoner bounced back after his first professional defeat - an memorable fight with Callum Smith in last winter - to claim a twentieth professional win.
Some ringside observers believed Parker, thirty-one, should have been awarded the decision but the contender endured the second defeat of his 28-match career.
Buatsi Claims Win in Slow Encounter
His career development has stalled in recent years through promotional disputes and extended periods of lack of fights.
Previously predicted as the next star of the British fight scene, he now stands trying to fulfil that early promise - perhaps in what feels like a last chance - under recent support.
He began cautiously, with the two boxers ready to overcommit in a cagey early stages.
Parker, whose sole prior loss came at super-middleweight against John Ryder in two years ago after suffering a hand injury mid-fight, was making just his second fight at the higher weight class.
By the halfway stage, Parker had made it competitive, causing problems for the favorite, who worked behind a tight guard to narrowly win the cleaner rounds.
The audience's tolerance wore thin because of regular grabbing, with the challenger going down various instances during close exchanges.
Parker finished the stronger, but multiple periods were difficult to score. Although, previous titleholder Carl Frampton - working with the broadcast network - had it particularly wide for Parker, awarding Buatsi just a single round.
The victor moves on, but with the caliber of power puncher Benavidez, and Russian star Dmitry Bivol dominating the weight class, the road to world glory - judging by this showing - still appears challenging.
Ricky Hatton Memorial Motivates Local Rivalry Battle
In the co-main event, Arthur beat Bradley Rea by split-decision in a terrific local rivalry fight to win the European light heavyweight belt.
The venue became quiet for a profoundly moving tribute before the fight as 10 bells sounded in memory of the city's native Hatton, who succumbed at age 46 in last month.
It was the initial significant broadcast match card in the location since the passing of Hatton. As the final chime ended, the quiet became a shout of "there's only one Ricky Hatton".
The action inside the ropes equaled the feeling. The eventual winner was sent to the canvas by a significant blow flush on the chin in the early round, getting up unsteadily to his footing on the count of eight.
But the red-colored shorts of Arthur soon pressed ahead again. From the other corner, the opponent - in rival City blue - was marked up quickly, noticeable damage forming under his face as the pair landed punishing blows.
The challenger, twenty-seven, connected with strong blows but Arthur, 34, matched the intensity with powerful upward punches as the action seldom decreased over twelve rounds.
Arthur edged the decision with official scores of 114-114, close margin and clearer margin, but the two athletes were persistent and courageous, everything their hero Hatton represented.
