British light-heavyweight Buatsi earned a win yet unimpressive start to life under new promoters with a points victory over Parker in Manchester.
The Olympic medal winner's activity level likely gained him a split decision in a competitive 10-round fight at the boxing arena.
But the challenger had his moments, delivering effective individual punches in a fight where the techniques failed to mesh, with excessive holding marring the action.
Two judges judged it 96-94 for to the winner, with the final judge a 95-95.
"It wasn't my greatest showing but how do champions respond? They prevail," Buatsi stated in the arena as he was faced boos from the spectators.
The British fighter bounced back after his initial pro loss - an thrilling contest with Callum Smith in February - to claim a twentieth professional win.
Some ringside observers thought Parker, 31, merited the victory but the British boxer experienced the second setback of his 28-bout career.
The Champion Secures Win in Slow Match
The fighter's advancement has slowed in recent years through contract problems and considerable time of inactivity.
Once tipped as the rising talent of the British fight scene, he now stands trying to achieve expectations - maybe in what seems like a final roll of the dice - under new promotional backing.
He began cautiously, with both competitors prepared to be aggressive in a cagey opening few rounds.
The challenger, whose single earlier setback came at the lower weight class against his former opponent in two years ago after breaking his hand in the contest, was making just his second bout at light-heavyweight.
By the fight's midpoint, Parker had stayed in contention, making life awkward for Buatsi, who utilized a protective stance to barely take the cleaner periods.
The spectators' acceptance decreased amid frequent holding, with the challenger going down several times during close exchanges.
The underdog closed well, but many rounds were difficult to score. Although, previous titleholder Carl Frampton - providing analysis for the broadcast network - had it particularly wide for Parker, awarding Buatsi just one round.
Buatsi moves on, but with the likes of hard hitter Benavidez, and Russian star Dmitry Bivol ruling the division, the journey to titles - judging by this showing - still looks steep.
Ricky Hatton Tribute Inspires Local Rivalry Battle
In the supporting bout, Lyndon Arthur overcame Bradley Rea by close verdict in a excellent Manchester vs Manchester contest to capture the continental belt.
The venue fell silent for a extremely emotional homage ahead of the contest as ten bells tolled in remembrance of Manchester's own Hatton, who succumbed at age 46 in September.
It was the premier televised boxing event in the city since the passing of Hatton. As the concluding sound disappeared, the quiet transitioned to a chant of "there's only one Ricky Hatton".
The fighting within the ring matched the emotion. Arthur was floored by a powerful punch directly to the jaw in the initial stage, standing uncertainly to his position on the referee's count.
But the red-colored shorts of Arthur soon advanced again. From the other corner, Rea - in opposing blue - was marked up soon, significant bruising developing under his eye as the fighters landed punishing blows.
Rea, 27, landed power punches but the champion, thirty-four, fought fire with fire with stinging uppercuts as the action rarely dropped over twelve rounds.
The victor edged the decision with official scores of even score, narrow lead and 115-112, but each competitor were persistent and courageous, everything their inspiration the legend stood for.
