British light-heavyweight Buatsi earned a win yet unimpressive start to his career under new promoters with a decision win over Parker in Manchester.
The Olympic medal winner's punch output seemingly secured him a majority decision in a close ten-round bout at the Manchester venue.
But the challenger found success, delivering effective clean blows in a match where the styles never quite clicked, with excessive holding marring the contest.
The scoring judges had it 96-94 to Buatsi, with the third a even score.
"It wasn't my finest display but what defines champions? They prevail," the victor said in the ring as he was met by negative reactions from the spectators.
The Londoner returned to winning ways after his initial pro loss - an memorable fight with Callum Smith in February - to secure a twentieth professional win.
Some ringside observers thought Parker, thirty-one, should have been awarded the win but the contender endured the second setback of his 28-fight career.
The Champion Secures Triumph in Uneventful Encounter
His career development has plateaued in recent times through management issues and considerable time of inactivity.
Previously predicted as the rising talent of the British fight scene, he now is positioned trying to realize his potential - perhaps in what appears to be a ultimate opportunity - under new promotional backing.
He opened tentatively, with both competitors prepared to be aggressive in a cagey opening few rounds.
Parker, whose single earlier setback came at super-middleweight against John Ryder in two years ago after suffering a hand injury mid-fight, was participating in his second fight at the higher weight class.
By the fight's midpoint, Parker had kept it close, creating difficulties for the favorite, who operated with a protective stance to barely take the more effective rounds.
The spectators' acceptance decreased amid regular grabbing, with Parker slipping to the canvas multiple occasions during grappling.
Parker closed well, but several stanzas were hard to judge. Nevertheless, former world champion Carl Frampton - working with the broadcast network - had it significantly in favor for Parker, scoring for Buatsi just only one stanza.
The victor moves on, but with the presence of heavy hitter Benavidez, and Russian star Bivol dominating the weight class, the journey to titles - based on this performance - still appears challenging.
Ricky Hatton Homage Fuels All-Manchester Contest
In the supporting bout, Lyndon Arthur overcame Bradley Rea by divided decision in a terrific all-Manchester battle to secure the continental title.
The arena grew hushed for a profoundly moving homage prior to the bout as a bell ceremony tolled in memory of local hero Ricky Hatton, who passed away at age 46 in recently.
It was the initial significant broadcast match card in the city since Hatton's death. As the concluding sound disappeared, the stillness became a shout of "there's only one Hatton".
The action inside the ropes matched the feeling. The eventual winner was floored by a powerful punch squarely on the face in the second round, getting up unsteadily to his feet on the eight count.
But the red-colored equipment of Arthur soon came forward again. From the other corner, Rea - in rival City blue - was injured early, heavy swelling developing under his eye as the pair landed hard punches.
Rea, twenty-seven, connected with strong blows but Arthur, 34, matched the intensity with powerful upward punches as the pace seldom decreased over twelve rounds.
Arthur edged the result with judges scorecards of draw, close margin and clearer margin, but both fighters were relentless and courageous, everything their hero the legend embodied.
