British light heavyweight Buatsi made a winning yet disappointing start to life under new promoters with a decision win over Parker in Manchester.
The Olympic medal winner's work rate likely gained him a majority decision in a tight 10-round contest at the Manchester venue.
But the challenger had his moments, connecting with powerful clean blows in a fight where the techniques didn't complement each other, with excessive holding marring the action.
Two judges scored it 96-94 for to the winner, with the final judge a 95-95 draw.
"It wasn't my finest display but what defines champions? They win," Buatsi said in the arena as he was greeted with negative reactions from the audience.
The Londoner returned to winning ways after his maiden career setback - an epic battle with Callum Smith in last winter - to earn a 20th career victory.
Some ringside observers felt the challenger, 31, deserved the win but the Derbyshire fighter suffered the second defeat of his 28-bout professional journey.
The Champion Secures Triumph in Dull Match
The fighter's advancement has slowed in recent years through contract problems and considerable time of inactivity.
Previously predicted as the rising talent of UK boxing, he now finds himself trying to realize his potential - possibly in what seems like a ultimate opportunity - under new promotional backing.
He opened tentatively, with the two boxers willing to take risks in a cagey initial rounds.
Parker, whose single earlier setback came at the lower weight class against his former opponent in two years ago after damaging his hand mid-fight, was competing in only his second bout at light-heavyweight.
By the halfway stage, the underdog had kept it close, causing problems for Buatsi, who worked behind a solid defense to narrowly win the more effective periods.
The crowd's patience decreased during frequent holding, with Parker falling to the floor several times during clinches.
The underdog finished the stronger, but many rounds were hard to judge. However, former world champion the boxing analyst - commentating on the broadcast network - had it strongly leaning for Parker, giving Buatsi just only one stanza.
Buatsi progresses, but with the likes of power puncher Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the path to championship success - based on this performance - still looks steep.
Ricky Hatton Memorial Motivates Local Rivalry Battle
In the supporting bout, Lyndon Arthur beat Rea by close verdict in a terrific local rivalry contest to secure the European light heavyweight championship.
The venue fell silent for a profoundly moving homage before the fight as a bell ceremony tolled in memory of the city's native Ricky Hatton, who succumbed at age 46 in September.
It was the first major televised fight night in the location since Hatton's passing. As the last bell disappeared, the silence gave way to a chant of "only one Hatton".
The action within the ring matched the emotion. The eventual winner was floored by a significant blow directly to the face in the early round, getting up unsteadily to his feet on the referee's count.
But the Manchester United-red attire of the fighter soon advanced again. From the other corner, Rea - in opposing blue - was damaged quickly, noticeable damage developing under his eye as the pair traded punishing blows.
Rea, 27, landed power punches but Arthur, 34, fought fire with fire with effective uppercuts as the action hardly diminished over the full distance.
Arthur barely secured the decision with judges scorecards of draw, 115-113 and 115-112, but each competitor were determined and brave, all the qualities their inspiration Hatton stood for.
