British light-heavyweight Buatsi made a winning yet unimpressive debut to his career under fresh promotional backing with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's punch output likely gained him a split decision in a close 10-round bout at the Co-op Live Arena.
But Parker had his moments, landing some telling individual punches in a contest where the techniques never quite clicked, with excessive holding spoiling the contest.
Two judges had it 96-94 for to the winner, with the final judge a even score.
"It wasn't my finest showing but how do champions respond? They win," Buatsi said in the ringside as he was faced negative reactions from the audience.
The Londoner got back to winning after his first professional defeat - an epic battle with Callum Smith in last winter - to secure a 20th pro success.
Various analysts felt Parker, 31, deserved the decision but the Derbyshire fighter suffered the second setback of his 28-fight career.
The Champion Earns Victory in Slow Affair
The fighter's advancement has plateaued in recent years through management issues and considerable time of inactivity.
Once tipped as the next star of UK boxing, he now stands trying to achieve expectations - maybe in what seems like a last chance - under fresh management.
He began cautiously, with both competitors ready to be aggressive in a tentative opening few rounds.
Parker, whose single earlier setback came at the lower weight class against John Ryder in two years ago after damaging his hand in the contest, was participating in his second bout at the higher weight class.
By the halfway stage, Parker had kept it close, making life awkward for the favorite, who operated with a tight guard to edge the cleaner rounds.
The audience's tolerance diminished amid regular grabbing, with the challenger falling to the floor several times during clinches.
The underdog ended stronger, but many rounds were difficult to score. However, ex-champion the boxing analyst - commentating on the broadcast network - had it strongly leaning for Parker, awarding Buatsi just only one stanza.
Buatsi advances, but with the caliber of heavy hitter Benavidez, and Russian champion Dmitry Bivol leading the category, the path to championship success - on this evidence - still seems difficult.
Ricky Hatton Tribute Fuels All-Manchester Fight
In the secondary fight, Lyndon Arthur defeated Rea by divided decision in a terrific Manchester vs Manchester battle to secure the continental belt.
The stadium grew hushed for a deeply poignant homage before the fight as a bell ceremony rang in memory of local hero Hatton, who passed away aged 46 in recently.
It was the premier televised match card in the location since Hatton's death. As the final chime faded, the quiet transitioned to a chant of "only one Ricky Hatton".
The fighting within the ring equaled the feeling. The eventual winner was knocked down by a significant blow flush on the face in the early round, getting up unsteadily to his feet on the referee's count.
But the Manchester United-red shorts of Arthur soon advanced again. On the opposite side, the opponent - in opposing blue - was marked up soon, noticeable damage appearing under his orbital area as the duo exchanged powerful shots.
Rea, twenty-seven, connected with strong blows but the champion, 34, matched the intensity with effective uppercuts as the pace seldom decreased over 12 rounds.
Arthur edged the result with official scores of draw, narrow lead and wider gap, but both fighters were relentless and full of heart, exactly what their hero Hatton represented.
