UK light heavyweight Buatsi earned a win yet underwhelming debut to his career under new promoters with a points victory over Parker in Manchester.
The Olympic medal winner's punch output seemingly secured him a majority decision in a competitive 10-round contest at the boxing arena.
But the challenger found success, landing some telling single shots in a contest where the techniques didn't complement each other, with excessive holding marring the fight.
The scoring judges scored it 96-94 in favor to Buatsi, with the remaining official a even score.
"It wasn't my greatest display but what do champions do? They win," the victor said in the ring as he was greeted with disapproval from the spectators.
The Londoner returned to winning ways after his first professional defeat - an thrilling contest with Callum Smith in last winter - to claim a twentieth professional win.
Various analysts felt the challenger, thirty-one, merited the win but the British boxer experienced the second loss of his 28-fight professional journey.
The Champion Earns Triumph in Slow Match
His career development has plateaued in the past few years through management issues and extended periods of lack of fights.
Previously predicted as the next star of British boxing, he now is positioned trying to fulfil that early promise - maybe in what appears to be a final roll of the dice - under fresh management.
He began cautiously, with the two boxers willing to overcommit in a cagey opening few rounds.
The challenger, whose single earlier setback came at super-middleweight against his former opponent in 2022 after damaging his hand during the bout, was participating in his second appearance at light-heavyweight.
By the halfway stage, Parker had kept it close, creating difficulties for Buatsi, who utilized a protective stance to narrowly win the more effective stanzas.
The spectators' acceptance wore thin during regular grabbing, with Parker going down various instances during close exchanges.
The underdog ended stronger, but multiple periods were hard to judge. Although, ex-champion Carl Frampton - commentating on Dazn - had it strongly leaning for Parker, scoring for Buatsi just a single round.
Buatsi progresses, but with the caliber of power puncher Benavidez, and Russian champion Bivol ruling the division, the path to championship success - based on this performance - still looks steep.
Hatton Tribute Fuels Manchester Derby Contest
In the co-main event, Arthur overcame Rea by divided decision in a excellent Manchester vs Manchester fight to capture the European light heavyweight belt.
The venue became quiet for a profoundly moving homage prior to the bout as 10 bells tolled in honor of the city's native Ricky Hatton, who died at age 46 in September.
It was the first major televised boxing event in the city since Hatton's death. As the final chime ended, the silence gave way to a shout of "only one Ricky Hatton".
The fighting within the ring matched the sentiment. Arthur was sent to the canvas by a powerful punch flush on the chin in the initial stage, standing uncertainly to his position on the eight count.
But the traditional red equipment of Arthur soon advanced again. From the other corner, Rea - in rival City blue - was injured quickly, noticeable damage appearing under his face as the duo traded punishing blows.
Rea, twenty-seven, delivered effective strikes but Arthur, 34, responded in kind with effective uppercuts as the intensity hardly diminished over the full distance.
Arthur narrowly won the decision with judges scorecards of 114-114, close margin and wider gap, but the two athletes were persistent and brave, all the qualities their idol Hatton stood for.
