British light heavyweight Buatsi secured a victory yet unimpressive debut to his career under new promoters with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's punch output appeared to earn him a majority decision in a close 10-round contest at the Manchester venue.
But the challenger landed significant shots, landing some telling single shots in a fight where the approaches never quite clicked, with frequent clinching disrupting the action.
The scoring judges had it 96-94 in favor to the winner, with the remaining official a even score.
"It wasn't my best display but what do champions do? They prevail," the victor stated in the ring as he was faced negative reactions from the audience.
The British fighter got back to winning after his initial pro loss - an thrilling contest with Callum Smith in February - to secure a 20th career victory.
Several boxing experts felt Parker, thirty-one, deserved the win but the British boxer endured the second defeat of his 28-match professional journey.
Buatsi Earns Triumph in Uneventful Match
The fighter's advancement has plateaued in recent years through promotional disputes and considerable time of inactivity.
Previously predicted as the rising talent of UK boxing, he now finds himself trying to realize his potential - possibly in what appears to be a last chance - under recent support.
He opened tentatively, with neither fighter willing to be aggressive in a tentative initial rounds.
Parker, whose only previous defeat came at super middleweight against his former opponent in two years ago after damaging his hand in the contest, was competing in only his second appearance at light heavyweight.
By the fight's midpoint, the underdog had kept it close, creating difficulties for Buatsi, who worked behind a protective stance to edge the more effective rounds.
The spectators' acceptance diminished because of frequent holding, with Parker slipping to the canvas various instances during grappling.
The underdog finished the stronger, but multiple periods were challenging to assess. However, previous titleholder Carl Frampton - working with Dazn - had it particularly wide for Parker, giving Buatsi just one round.
The victor progresses, but with the presence of heavy hitter Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the road to world glory - on this evidence - still seems difficult.
Ricky Hatton Homage Fuels All-Manchester Fight
In the co-main event, Lyndon Arthur beat Rea by close verdict in a outstanding all-Manchester battle to secure the European light heavyweight championship.
The arena became quiet for a extremely emotional homage before the fight as ten bells tolled in remembrance of local hero Hatton, who died aged 46 in last month.
It was the first major televised boxing event in the urban center since Hatton's death. As the final chime ended, the silence transitioned to a chant of "only one Hatton".
The action within the ring reflected the emotion. Arthur was knocked down by a significant blow squarely on the jaw in the second round, getting up unsteadily to his position on the count of eight.
But the traditional red equipment of Arthur soon came forward again. On the opposite side, Rea - in opposing blue - was marked up quickly, significant bruising appearing under his orbital area as the fighters exchanged punishing blows.
Rea, twenty-seven, delivered effective strikes but the champion, 34, responded in kind with effective uppercuts as the intensity hardly diminished over the full distance.
Arthur narrowly won the decision with official scores of 114-114, narrow lead and wider gap, but each competitor were determined and brave, exactly what their hero Hatton represented.
