UK light heavyweight Buatsi secured a victory yet unimpressive beginning to life under new promoters with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's activity level seemingly secured him a majority decision in a tight ten-round fight at the Co-op Live Arena.
But the challenger had his moments, landing some telling individual punches in a contest where the styles never quite clicked, with constant grabbing disrupting the fight.
Two judges scored it 96-94 for to the winner, with the final judge a 95-95 draw.
"It wasn't my greatest performance but what do champions do? They win," Buatsi said in the ringside as he was met by negative reactions from the crowd.
The British fighter returned to winning ways after his maiden career setback - an thrilling contest with Callum Smith in February - to secure a 20th pro success.
Several boxing experts thought Parker, 31, deserved the decision but the British boxer endured the second loss of his 28-bout career.
The Champion Earns Triumph in Uneventful Affair
The fighter's advancement has slowed in recent years through contract problems and long spells of inactivity.
Previously predicted as the next star of UK boxing, he now is positioned trying to fulfil that early promise - perhaps in what feels like a last chance - under recent support.
He opened tentatively, with the two boxers prepared to overcommit in a tentative initial rounds.
The challenger, whose only previous defeat came at the lower weight class against John Ryder in two years ago after breaking his hand mid-fight, was competing in only his second bout at light-heavyweight.
By the fight's midpoint, the underdog had made it competitive, creating difficulties for the favorite, who worked behind a tight guard to edge the more effective rounds.
The spectators' acceptance decreased during frequent holding, with the challenger falling to the floor various instances during grappling.
The underdog ended stronger, but multiple periods were challenging to assess. Although, previous titleholder the boxing analyst - working with the broadcast network - had it particularly wide for Parker, scoring for Buatsi just a single round.
The victor moves on, but with the presence of power puncher Benavidez, and Russian star Dmitry Bivol leading the category, the path to championship success - based on this performance - still appears challenging.
Hatton Homage Inspires Local Rivalry Battle
In the secondary fight, Lyndon Arthur defeated Rea by close verdict in a excellent local rivalry battle to win the European light-heavyweight belt.
The venue fell silent for a deeply poignant tribute before the fight as 10 bells sounded in memory of the city's native Hatton, who succumbed 46 years old in September.
It was the premier televised fight night in the location since Hatton's death. As the concluding sound faded, the quiet gave way to a chant of "there's only one Hatton".
The fighting within the ring matched the emotion. The eventual winner was sent to the canvas by a powerful punch flush on the face in the second round, getting up unsteadily to his position on the referee's count.
But the traditional red attire of Arthur soon advanced again. From the other corner, Rea - in rival City blue - was damaged soon, significant bruising developing under his eye as the fighters traded hard punches.
Rea, twenty-seven, connected with strong blows but Arthur, 34, matched the intensity with stinging uppercuts as the intensity rarely dropped over twelve rounds.
Arthur edged the verdict with scoring tallies of 114-114, close margin and 115-112, but each competitor were persistent and courageous, all the qualities their idol the legend embodied.
