UK light-heavyweight Buatsi secured a victory yet disappointing beginning to his career under fresh promotional backing with a points victory over Parker in the Manchester arena.
The Olympic medal winner's work rate seemingly secured him a majority decision in a tight 10-round contest at the Co-op Live Arena.
But Parker landed significant shots, delivering effective clean blows in a contest where the styles failed to mesh, with excessive holding spoiling the action.
The scoring judges had it 96-94 in favor to the winner, with the third a even score.
"It wasn't my best performance but what do champions do? They succeed," the victor commented in the arena as he was met by disapproval from the audience.
The British fighter bounced back after his initial pro loss - an memorable fight with his previous opponent in last winter - to claim a twentieth professional win.
Some ringside observers believed the challenger, thirty-one, merited the decision but the British boxer suffered the second defeat of his 28-match professional journey.
The Champion Claims Win in Slow Affair
Buatsi's progress has plateaued in the past few years through contract problems and extended periods of inactivity.
Earlier forecast as the next star of UK boxing, he now finds himself trying to fulfil that early promise - possibly in what appears to be a final roll of the dice - under fresh management.
He started carefully, with neither fighter ready to take risks in a cagey opening few rounds.
Parker, whose sole prior loss came at super middleweight against John Ryder in 2022 after damaging his hand during the bout, was competing in only his second fight at light-heavyweight.
By the middle rounds, Parker had kept it close, creating difficulties for Buatsi, who utilized a tight guard to edge the cleaner periods.
The spectators' acceptance decreased during frequent holding, with the challenger going down various instances during clinches.
Parker finished the stronger, but many rounds were hard to judge. However, former world champion the boxing analyst - providing analysis for the broadcast network - had it significantly in favor for Parker, giving Buatsi just only one stanza.
The victor advances, but with the presence of heavy hitter Benavidez, and Eastern European fighter Bivol ruling the division, the road to world glory - on this evidence - still appears challenging.
Hatton Memorial Motivates All-Manchester Contest
In the secondary fight, Lyndon Arthur overcame Rea by split-decision in a terrific all-Manchester fight to capture the continental belt.
The arena became quiet for a deeply poignant tribute prior to the bout as a bell ceremony tolled in honor of the city's native Hatton, who passed away aged 46 in last month.
It was the premier televised fight night in the city since Hatton's passing. As the final chime faded, the stillness became a shout of "there's just one Ricky Hatton".
The action during the bout reflected the sentiment. Arthur was knocked down by a left hook squarely on the chin in the early round, rising shakily to his feet on the referee's count.
But the red-colored attire of the fighter soon pressed ahead again. From the other corner, Rea - in competitive blue - was damaged early, noticeable damage developing under his eye as the fighters landed hard punches.
Rea, 27, connected with strong blows but the champion, thirty-four, fought fire with fire with effective uppercuts as the pace rarely dropped over twelve rounds.
The victor barely secured the result with scoring tallies of draw, close margin and clearer margin, but both fighters were determined and full of heart, all the qualities their hero Hatton represented.
