UK light heavyweight Buatsi earned a win yet underwhelming debut to life under recent management with a decision win over Parker in Manchester.
The Olympic medal winner's punch output likely gained him a judges' verdict in a close 10-round bout at the Co-op Live Arena.
But Parker found success, connecting with powerful single shots in a contest where the techniques didn't complement each other, with constant grabbing marring the fight.
Two judges had it 96-94 in favor to the winner, with the remaining official a even score.
"It wasn't my greatest performance but how do champions respond? They win," Buatsi commented in the ring as he was greeted with disapproval from the audience.
The British fighter got back to winning after his maiden career setback - an thrilling contest with his previous opponent in last winter - to secure a 20th pro success.
Various analysts believed Parker, 31, should have been awarded the victory but the British boxer experienced the second setback of his 28-match career.
Buatsi Claims Triumph in Slow Match
The fighter's advancement has plateaued in recent years through contract problems and considerable time of lack of fights.
Earlier forecast as the future of UK boxing, he now finds himself trying to fulfil that early promise - maybe in what feels like a ultimate opportunity - under recent support.
He started carefully, with both competitors willing to take risks in a calculated early stages.
The challenger, whose sole prior loss came at super middleweight against John Ryder in two years ago after damaging his hand mid-fight, was competing in only his second appearance at light heavyweight.
By the fight's midpoint, Parker had stayed in contention, causing problems for the favorite, who utilized a tight guard to edge the cleaner stanzas.
The crowd's patience wore thin amid constant clinching, with Parker going down several times during close exchanges.
Parker ended stronger, but multiple periods were difficult to score. Nevertheless, ex-champion Carl Frampton - providing analysis for the broadcast network - had it strongly leaning for the challenger, giving Buatsi just a single round.
Buatsi moves on, but with the presence of heavy hitter Benavidez, and Russian champion Bivol ruling the division, the path to championship success - on this evidence - still seems difficult.
Ricky Hatton Memorial Inspires Local Rivalry Fight
In the co-main event, Arthur beat Rea by close verdict in a terrific local rivalry fight to secure the continental belt.
The venue grew hushed for a deeply poignant memorial ahead of the contest as ten bells rang in memory of local hero Ricky Hatton, who passed away aged 46 in recently.
It was the first major televised match card in the location since Hatton's passing. As the final chime disappeared, the quiet gave way to a shout of "there's only one Hatton".
The competition inside the ropes reflected 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 the fighter soon advanced again. On the opposite side, the opponent - in opposing blue - was marked up early, heavy swelling developing under his eye as the duo landed punishing blows.
Rea, 27, delivered effective strikes but the champion, thirty-four, responded in kind with stinging uppercuts as the pace seldom decreased over 12 rounds.
The victor edged the result with judges scorecards of 114-114, narrow lead and 115-112, but the two athletes were determined and courageous, all the qualities their inspiration the legend represented.
