British light heavyweight Buatsi earned a win yet disappointing start to his career under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic medal winner's activity level seemingly secured him a split decision in a tight 10-round contest at the boxing arena.
But the challenger landed significant shots, connecting with powerful clean blows in a fight where the approaches never quite clicked, with excessive holding marring the fight.
The officials judged it 96-94 to Buatsi, with the remaining official a 95-95.
"It wasn't my greatest display but how do champions respond? They prevail," the victor stated in the ring as he was met by negative reactions from the audience.
The Londoner got back to winning after his maiden career setback - an thrilling contest with his previous opponent in last winter - to claim a 20th career victory.
Some ringside observers thought Parker, thirty-one, merited the decision but the Derbyshire fighter endured the second loss of his 28-fight career.
The Champion Earns Win in Dull Match
The fighter's advancement has stalled in the past few years through contract problems and long spells of lack of fights.
Earlier forecast as the next star of the British fight scene, he now stands trying to realize his potential - possibly in what feels like a ultimate opportunity - under new promotional backing.
He opened tentatively, with both competitors ready to be aggressive in a cagey opening few rounds.
The challenger, whose single earlier setback 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 the higher weight class.
By the middle rounds, Parker had kept it close, making life awkward for Buatsi, who utilized a solid defense to barely take the better periods.
The audience's tolerance diminished during constant clinching, with Parker falling to the floor several times during grappling.
Parker closed well, but multiple periods were hard to judge. Although, previous titleholder the boxing analyst - working with the broadcast network - had it strongly leaning for the challenger, awarding Buatsi just one round.
The victor moves on, but with the caliber of power puncher Benavidez, and Russian champion Dmitry Bivol leading the category, the journey to titles - judging by this showing - still looks steep.
Hatton Tribute Inspires All-Manchester Contest
In the secondary fight, Lyndon Arthur beat Rea by divided decision in a excellent Manchester vs Manchester contest to win the European light-heavyweight belt.
The arena fell silent for a deeply poignant tribute ahead of the contest as 10 bells tolled in memory of Manchester's own Hatton, who succumbed at age 46 in recently.
It was the premier televised boxing event in the urban center since Hatton's death. As the final chime disappeared, the quiet became a shout of "there's only one Ricky Hatton".
The competition during the bout equaled the sentiment. Arthur was floored by a left hook squarely on the jaw in the initial stage, getting up unsteadily to his feet on the count of eight.
But the traditional red shorts of Arthur soon came forward again. Across the ring, the opponent - in opposing blue - was marked up soon, noticeable damage forming under his eye as the duo traded powerful shots.
Rea, twenty-seven, connected with strong blows but Arthur, 34, responded in kind with stinging uppercuts as the action seldom decreased over the full distance.
The victor edged the result with official scores of 114-114, narrow lead and 115-112, but each competitor were relentless and full of heart, exactly what their inspiration the legend embodied.
