UK light-heavyweight Buatsi made a winning yet underwhelming beginning to life under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output seemingly secured him a split decision in a competitive ten-round fight at the Manchester venue.
But the challenger found success, landing some telling individual punches in a fight where the techniques failed to mesh, with excessive holding marring the action.
The officials judged it 96-94 in favor to the winner, with the remaining official a 95-95.
"It wasn't my best performance but how do champions respond? They succeed," the victor commented in the arena as he was met by disapproval from the spectators.
The Londoner returned to winning ways after his maiden career setback - an thrilling contest with Callum Smith in February - to claim a 20th pro success.
Various analysts believed Parker, 31, merited the decision but the contender experienced the second loss of his 28-match professional journey.
Buatsi Earns Win in Dull Encounter
His career development has plateaued in recent years through management issues and long spells of lack of fights.
Earlier forecast as the future of British boxing, he now is positioned trying to achieve expectations - maybe in what feels like a final roll of the dice - under fresh management.
He started carefully, with neither fighter prepared to overcommit in a cagey early stages.
The challenger, whose only previous defeat came at super middleweight against his former opponent in two years ago after breaking his hand in the contest, was participating in his second appearance at light heavyweight.
By the fight's midpoint, Parker had stayed in contention, making life awkward for the favorite, who worked behind a tight guard to narrowly win the better stanzas.
The audience's tolerance diminished amid regular grabbing, with the challenger going down multiple occasions during close exchanges.
Parker closed well, but multiple periods were difficult to score. However, former world champion Carl Frampton - working with the broadcast network - had it strongly leaning for Parker, scoring for Buatsi just a single round.
Buatsi progresses, but with the caliber of hard hitter Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the path to championship success - based on this performance - still appears challenging.
Hatton Memorial Motivates Manchester Derby Battle
In the supporting bout, Arthur beat Bradley Rea by close verdict in a terrific local rivalry fight to secure the continental belt.
The arena grew hushed for a extremely emotional memorial prior to the bout as ten bells tolled in memory of the city's native Hatton, who died 46 years old in recently.
It was the initial significant broadcast boxing event in the city since the passing of Hatton. As the last bell ended, the quiet became a chant of "there's just one Ricky Hatton".
The competition inside the ropes equaled the feeling. The eventual winner was knocked down by a powerful punch squarely on the chin in the second round, standing uncertainly to his footing on the count of eight.
But the traditional red equipment of Arthur soon came forward again. From the other corner, Rea - in rival City blue - was damaged soon, significant bruising forming under his eye as the pair traded punishing blows.
The challenger, 27, delivered effective strikes but the champion, thirty-four, responded in kind with effective uppercuts as the action rarely dropped over the full distance.
The victor edged the verdict with scoring tallies of draw, close margin and clearer margin, but both fighters were persistent and full of heart, all the qualities their idol the legend stood for.
