British light-heavyweight Buatsi secured a victory yet disappointing start to life under recent management with a unanimous decision over Parker in Manchester.
The Olympic medal winner's work rate seemingly secured him a split decision in a close ten-round bout at the Co-op Live Arena.
But the challenger found success, delivering effective single shots in a contest where the techniques never quite clicked, with constant grabbing spoiling the fight.
Two judges scored it 96-94 for to the winner, with the third a 95-95 draw.
"It wasn't my best performance but how do champions respond? They prevail," the victor commented in the ring as he was greeted with disapproval from the spectators.
The British fighter bounced back after his maiden career setback - an thrilling contest with Callum Smith in February - to secure a 20th pro success.
Some ringside observers believed the challenger, thirty-one, should have been awarded the victory but the contender experienced the second loss of his 28-bout career.
The Champion Claims Win in Slow Encounter
His career development has stalled in recent years through promotional disputes and extended periods of lack of fights.
Previously predicted as the rising talent of UK boxing, he now stands trying to realize his potential - perhaps in what feels like a ultimate opportunity - under fresh management.
He began cautiously, with both competitors ready to take risks in a cagey initial rounds.
Parker, whose only previous defeat came at super middleweight against his former opponent in two years ago after damaging his hand during the bout, was participating in his second appearance at light heavyweight.
By the halfway stage, the underdog had stayed in contention, making life awkward for Buatsi, who utilized a solid defense to edge the better periods.
The audience's tolerance diminished because of constant clinching, with Parker going down several times during clinches.
Parker ended stronger, but multiple periods were challenging to assess. However, ex-champion Carl Frampton - working with the broadcast network - had it particularly wide for Parker, scoring for Buatsi just only one stanza.
Buatsi progresses, but with the likes of heavy hitter Benavidez, and Russian star Dmitry Bivol ruling the division, the path to championship success - judging by this showing - still seems difficult.
Hatton Tribute Inspires Local Rivalry Battle
In the secondary fight, Arthur overcame Rea by split-decision in a excellent local rivalry battle to win the continental belt.
The venue grew hushed for a extremely emotional homage before the fight as 10 bells rang in memory of Manchester's own Hatton, who succumbed aged 46 in September.
It was the first major televised match card in the location since Hatton's death. As the concluding sound faded, the stillness transitioned to a shout of "there's just one Hatton".
The competition inside the ropes equaled the feeling. Arthur was knocked down by a powerful punch squarely on the jaw in the second round, standing uncertainly to his feet on the count of eight.
But the traditional red equipment of the fighter soon advanced again. Across the ring, the opponent - in rival City blue - was damaged early, noticeable damage developing under his face as the pair traded powerful shots.
Rea, twenty-seven, delivered effective strikes but the champion, thirty-four, responded in kind with stinging uppercuts as the pace seldom decreased over the full distance.
The victor edged the verdict with scoring tallies of draw, narrow lead and 115-112, but the two athletes were determined and full of heart, exactly what their idol Hatton embodied.
