British light-heavyweight Buatsi made a winning yet disappointing start to life under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic bronze medallist's work rate appeared to earn him a majority decision in a competitive 10-round contest at the Manchester venue.
But the challenger found success, delivering effective single shots in a contest where the approaches never quite clicked, with frequent clinching spoiling the fight.
Two judges had it 96-94 for to the winner, with the remaining official a 95-95.
"It wasn't my best display but how do champions respond? They succeed," the victor stated in the ringside as he was faced boos from the spectators.
The Londoner got back to winning after his maiden career setback - an epic battle with Callum Smith in February - to secure a 20th pro success.
Various analysts felt Parker, thirty-one, merited the win but the Derbyshire fighter endured the second loss of his 28-bout career.
The Champion Earns Win in Dull Encounter
Buatsi's progress has slowed in recent times through promotional disputes and extended periods of inactivity.
Once tipped as the rising talent of British boxing, he now finds himself trying to achieve expectations - possibly in what appears to be a ultimate opportunity - under fresh management.
He opened tentatively, with the two boxers ready to take risks in a cagey early stages.
Parker, whose single earlier setback came at super middleweight against John Ryder in two years ago after suffering a hand injury in the contest, was making just his second bout at light heavyweight.
By the halfway stage, the underdog had stayed in contention, causing problems for Buatsi, who operated with a solid defense to edge the cleaner rounds.
The crowd's patience diminished because of constant clinching, with Parker falling to the floor several times during close exchanges.
The underdog ended stronger, but many rounds were hard to judge. Nevertheless, previous titleholder Carl Frampton - working with the broadcast network - had it significantly in favor for Parker, giving Buatsi just a single round.
The victor progresses, but with the caliber of hard hitter Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the journey to titles - judging by this showing - still seems difficult.
Hatton Memorial Motivates Local Rivalry Contest
In the secondary fight, Arthur overcame Rea by close verdict in a outstanding local rivalry fight to win the European light-heavyweight belt.
The venue fell silent for a extremely emotional homage before the fight as a bell ceremony tolled in remembrance of the city's native Hatton, who passed away 46 years old in September.
It was the premier televised fight night in the location since the passing of Hatton. As the last bell faded, the silence transitioned to a roar of "there's only one Ricky Hatton".
The competition inside the ropes matched the sentiment. The eventual winner was floored by a significant blow directly to the chin in the second round, getting up unsteadily to his footing on the eight count.
But the Manchester United-red equipment of Arthur soon advanced again. On the opposite side, Rea - in rival City blue - was marked up early, significant bruising forming under his orbital area as the fighters exchanged hard punches.
The challenger, twenty-seven, landed power punches but the champion, thirty-four, matched the intensity with powerful upward punches as the intensity seldom decreased over 12 rounds.
The victor edged the decision with official scores of draw, narrow lead and wider gap, but each competitor were relentless and brave, all the qualities their inspiration the legend stood for.
