British light heavyweight Buatsi secured a victory yet unimpressive debut to life under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic medal winner's activity level likely gained him a judges' verdict in a tight 10-round bout at the Manchester venue.
But Parker landed significant shots, landing some telling single shots in a fight where the styles failed to mesh, with excessive holding spoiling the fight.
The scoring judges scored it 96-94 for to Buatsi, with the third a even score.
"It wasn't my best performance but what do champions do? They succeed," the victor said in the ringside as he was greeted with disapproval from the audience.
The Londoner returned to winning ways after his initial pro loss - an memorable fight with Callum Smith in February - to earn a 20th career victory.
Various analysts felt Parker, thirty-one, deserved the decision but the British boxer suffered the second loss of his 28-fight career.
Buatsi Claims Triumph in Dull Match
His career development has stalled in recent times through contract problems and long spells of lack of fights.
Earlier forecast as the next star of UK boxing, he now finds himself trying to realize his potential - possibly in what feels like a ultimate opportunity - under recent support.
He opened tentatively, with both competitors ready to overcommit in a cagey initial rounds.
Parker, whose sole prior loss came at super middleweight against John Ryder in two years ago after breaking his hand during the bout, was making just his second fight at light heavyweight.
By the fight's midpoint, the underdog had made it competitive, causing problems for Buatsi, who utilized a solid defense to narrowly win the cleaner rounds.
The spectators' acceptance decreased amid frequent holding, with Parker going down several times during grappling.
Parker finished the stronger, but multiple periods were challenging to assess. However, former world champion the boxing analyst - providing analysis for Dazn - had it particularly wide for the challenger, awarding Buatsi just one round.
Buatsi advances, but with the presence of heavy hitter Benavidez, and Russian champion Bivol dominating the weight class, the path to championship success - judging by this showing - still appears challenging.
Ricky Hatton Tribute Fuels All-Manchester Fight
In the secondary fight, Lyndon Arthur defeated Bradley Rea by close verdict in a outstanding all-Manchester battle to win the European light-heavyweight title.
The stadium grew hushed for a deeply poignant homage before the fight as ten bells rang in honor of the city's native Ricky Hatton, who passed away aged 46 in September.
It was the initial significant broadcast fight night in the urban center since the passing of Hatton. As the final chime ended, the stillness gave way to a shout of "there's just one Hatton".
The action within the ring reflected the sentiment. Arthur was floored by a significant blow flush on the chin in the initial stage, getting up unsteadily to his position on the referee's count.
But the traditional red shorts of Arthur soon came forward again. On the opposite side, the opponent - in competitive blue - was injured quickly, noticeable damage forming under his eye as the duo landed punishing blows.
Rea, twenty-seven, delivered effective strikes but Arthur, 34, responded in kind with powerful upward punches as the pace seldom decreased over the full distance.
Arthur barely secured the result with scoring tallies of even score, close margin and 115-112, but the two athletes were relentless and brave, everything their idol Hatton represented.
