British light heavyweight Buatsi earned a win yet underwhelming start to his career under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's punch output likely gained him a judges' verdict in a close ten-round bout at the Manchester venue.
But the challenger had his moments, connecting with powerful individual punches in a fight where the techniques failed to mesh, with excessive holding disrupting the action.
The scoring judges scored it 96-94 in favor to Buatsi, with the final judge a 95-95 draw.
"It wasn't my best performance but what do champions do? They prevail," the victor commented in the arena as he was met by boos from the audience.
The British fighter got back to winning after his initial pro loss - an memorable fight with Callum Smith in February - to earn a twentieth professional win.
Various analysts felt the challenger, 31, merited the win but the Derbyshire fighter suffered the second defeat of his 28-match career.
Buatsi Secures Win in Uneventful Encounter
The fighter's advancement has plateaued in recent times through promotional disputes and extended periods of inactivity.
Previously predicted as the rising talent of the British fight scene, he now is positioned trying to achieve expectations - possibly in what feels like a final roll of the dice - under fresh management.
He began cautiously, with both competitors willing to overcommit in a calculated opening few rounds.
Parker, whose sole prior loss came at super-middleweight against his former opponent in 2022 after breaking his hand in the contest, was making just his second fight at light heavyweight.
By the middle rounds, the underdog had stayed in contention, causing problems for the favorite, who worked behind a tight guard to edge the better periods.
The crowd's patience diminished amid regular grabbing, with the challenger slipping to the canvas various instances during close exchanges.
The underdog finished the stronger, but many rounds were difficult to score. However, previous titleholder Carl Frampton - working with the broadcast network - had it strongly leaning for the challenger, awarding Buatsi just one round.
Buatsi moves on, but with the presence of hard hitter Benavidez, and Russian star Dmitry Bivol dominating the weight class, the road to world glory - judging by this showing - still seems difficult.
Hatton Homage Motivates All-Manchester Contest
In the co-main event, Lyndon Arthur defeated Bradley Rea by split-decision in a excellent all-Manchester battle to win the continental championship.
The venue grew hushed for a extremely emotional homage prior to the bout as ten bells tolled in honor of local hero Ricky Hatton, who passed away 46 years old in last month.
It was the premier televised match card in the urban center since the passing of Hatton. As the concluding sound ended, the silence gave way to a roar of "there's just one Hatton".
The action inside the ropes equaled the emotion. The eventual winner was knocked down by a significant blow squarely on the face in the early round, rising shakily to his feet on the eight count.
But the Manchester United-red equipment of Arthur soon advanced again. Across the ring, Rea - in rival City blue - was injured quickly, heavy swelling forming under his eye as the pair exchanged powerful shots.
Rea, 27, landed power punches but the champion, thirty-four, fought fire with fire with stinging uppercuts as the intensity hardly diminished over the full distance.
The victor edged the result with scoring tallies of draw, 115-113 and 115-112, but both fighters were relentless and full of heart, all the qualities their hero Hatton represented.
