British light heavyweight Buatsi made a winning yet disappointing debut to his career under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's work rate likely gained him a majority decision in a tight 10-round fight at the boxing arena.
But the challenger landed significant shots, delivering effective individual punches in a match where the approaches didn't complement each other, with constant grabbing disrupting the action.
The scoring judges scored it 96-94 to Buatsi, with the remaining official a even score.
"It wasn't my best showing but what do champions do? They succeed," the victor commented in the arena as he was faced negative reactions from the spectators.
The British fighter returned to winning ways after his first professional defeat - an memorable fight with Callum Smith in February - to claim a 20th career victory.
Some ringside observers felt Parker, 31, should have been awarded the decision but the British boxer endured the second setback of his 28-bout professional journey.
Buatsi Secures Triumph in Slow Match
The fighter's advancement has plateaued in recent times through contract problems and extended periods of inactivity.
Previously predicted as the rising talent of UK boxing, he now finds himself trying to achieve expectations - maybe in what appears to be a final roll of the dice - under new promotional backing.
He started carefully, with both competitors prepared to overcommit in a calculated opening few rounds.
The challenger, whose sole prior loss came at super-middleweight against his former opponent in two years ago after breaking his hand mid-fight, was making just his second appearance at light-heavyweight.
By the halfway stage, Parker had stayed in contention, creating difficulties for Buatsi, who utilized a tight guard to edge the cleaner rounds.
The audience's tolerance wore thin because of frequent holding, with the challenger going down several times during close exchanges.
Parker closed well, but multiple periods were hard to judge. Nevertheless, previous titleholder Carl Frampton - working with Dazn - had it significantly in favor for the challenger, awarding Buatsi just one round.
Buatsi progresses, but with the caliber of power puncher Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the path to championship success - based on this performance - still appears challenging.
Ricky Hatton Memorial Inspires All-Manchester Battle
In the supporting bout, Arthur overcame Rea by divided decision in a terrific all-Manchester battle to secure the continental belt.
The arena became quiet for a deeply poignant homage ahead of the contest as a bell ceremony tolled in memory of the city's native Hatton, who passed away aged 46 in September.
It was the initial significant broadcast boxing event in the city since the passing of Hatton. As the concluding sound faded, the stillness became a chant of "only one Ricky Hatton".
The fighting during the bout reflected the feeling. Arthur was sent to the canvas by a powerful punch squarely on the face in the initial stage, getting up unsteadily to his position on the count of eight.
But the red-colored attire of the fighter soon came forward again. On the opposite side, Rea - in rival City blue - was damaged soon, significant bruising appearing under his orbital area as the duo traded hard punches.
The challenger, 27, delivered effective strikes but Arthur, thirty-four, matched the intensity with stinging uppercuts as the pace rarely dropped over the full distance.
Arthur barely secured the verdict with judges scorecards of 114-114, 115-113 and clearer margin, but each competitor were relentless and brave, everything their inspiration the legend embodied.
