UK light-heavyweight Buatsi secured a victory yet underwhelming start to life under new promoters with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's punch output seemingly secured him a majority decision in a competitive ten-round contest at the Co-op Live Arena.
But the challenger had his moments, connecting with powerful single shots in a match where the styles failed to mesh, with constant grabbing marring the fight.
The scoring judges had it 96-94 for to Buatsi, with the third a 95-95 draw.
"It wasn't my greatest showing but how do champions respond? They succeed," the victor stated in the arena as he was met by disapproval from the spectators.
The Londoner got back to winning after his first professional defeat - an memorable fight with his previous opponent in February - to claim a 20th pro success.
Some ringside observers felt Parker, 31, merited the decision but the British boxer endured the second defeat of his 28-match career.
The Champion Earns Triumph in Dull Match
The fighter's advancement has slowed in recent years through management issues and considerable time of lack of fights.
Earlier forecast as the future of the British fight scene, he now stands trying to realize his potential - perhaps in what feels like a ultimate opportunity - under fresh management.
He began cautiously, with the two boxers prepared to be aggressive in a calculated early stages.
The challenger, whose single earlier setback came at super-middleweight against his former opponent in two years ago after suffering a hand injury in the contest, was making just his second fight at light heavyweight.
By the halfway stage, Parker had stayed in contention, making life awkward for the favorite, who operated with a protective stance to barely take the better stanzas.
The spectators' acceptance decreased because of regular grabbing, with Parker falling to the floor multiple occasions during close exchanges.
Parker finished the stronger, but several stanzas were difficult to score. However, ex-champion Carl Frampton - working with Dazn - had it particularly wide for the challenger, scoring for Buatsi just a single round.
Buatsi advances, but with the presence 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 Homage Fuels Manchester Derby Fight
In the supporting bout, Lyndon Arthur defeated Rea by close verdict in a terrific local rivalry contest to capture the continental championship.
The stadium grew hushed for a profoundly moving homage ahead of the contest as 10 bells tolled in remembrance of local hero Ricky Hatton, who succumbed 46 years old in recently.
It was the premier televised match card in the location since Hatton's death. As the final chime ended, the silence gave way to a roar of "there's just one Ricky Hatton".
The action during the bout equaled the sentiment. Arthur was sent to the canvas by a left hook squarely on the jaw in the initial stage, standing uncertainly to his footing on the eight count.
But the traditional red shorts of Arthur soon came forward again. On the opposite side, the opponent - in competitive blue - was marked up soon, heavy swelling forming under his eye as the duo exchanged punishing blows.
The challenger, 27, connected with strong blows but Arthur, 34, matched the intensity with stinging uppercuts as the pace hardly diminished over 12 rounds.
The victor barely secured the verdict with official scores of draw, narrow lead and clearer margin, but the two athletes were determined and full of heart, everything their idol Hatton represented.
