British light-heavyweight Buatsi earned a win yet disappointing debut to life under recent management with a decision win over Parker in the Manchester arena.
The Olympic medal winner's work rate seemingly secured him a split decision in a tight ten-round contest at the Manchester venue.
But Parker landed significant shots, delivering effective single shots in a fight where the techniques didn't complement each other, with constant grabbing marring the contest.
The officials judged it 96-94 to the winner, with the remaining official a 95-95 draw.
"It wasn't my finest performance but what defines champions? They prevail," the victor commented in the ring as he was met by boos from the crowd.
The British fighter returned to winning ways after his initial pro loss - an memorable fight with Callum Smith in last winter - to claim a 20th pro success.
Various analysts felt Parker, 31, should have been awarded the decision but the contender experienced the second loss of his 28-match professional journey.
The Champion Earns Triumph in Uneventful Encounter
The fighter's advancement has slowed in recent times through promotional disputes and considerable time of inactivity.
Earlier forecast as the next star of UK boxing, he now finds himself trying to achieve expectations - possibly in what appears to be a ultimate opportunity - under new promotional backing.
He opened tentatively, with the two boxers prepared to overcommit in a cagey early stages.
The challenger, whose sole prior loss came at the lower weight class against John Ryder in 2022 after breaking his hand in the contest, was competing in only his second bout at light-heavyweight.
By the middle rounds, Parker had kept it close, making life awkward for the favorite, who operated with a solid defense to narrowly win the more effective periods.
The crowd's patience diminished because of frequent holding, with the challenger falling to the floor various instances during grappling.
The underdog closed well, but several stanzas were hard to judge. However, former world champion the boxing analyst - working with Dazn - had it significantly in favor for Parker, scoring for Buatsi just a single round.
Buatsi moves on, but with the likes of power puncher Benavidez, and Russian champion Bivol ruling the division, the journey to titles - on this evidence - still seems difficult.
Ricky Hatton Tribute Fuels Manchester Derby Fight
In the supporting bout, Lyndon Arthur defeated Bradley Rea by divided decision in a outstanding all-Manchester fight to win the European light-heavyweight title.
The arena grew hushed for a extremely emotional homage before the fight as ten bells sounded in remembrance of the city's native Ricky Hatton, who succumbed aged 46 in last month.
It was the initial significant broadcast boxing event in the urban center since Hatton's passing. As the final chime ended, the stillness became a shout of "there's just one Hatton".
The fighting during the bout reflected the sentiment. Arthur was sent to the canvas by a left hook directly to the chin in the second round, standing uncertainly to his feet on the referee's count.
But the Manchester United-red attire of the fighter soon pressed ahead again. On the opposite side, the opponent - in rival City blue - was damaged soon, heavy swelling developing under his eye as the duo exchanged hard punches.
The challenger, twenty-seven, delivered effective strikes but the champion, thirty-four, matched the intensity with powerful upward punches as the action hardly diminished over the full distance.
Arthur edged the verdict with judges scorecards of even score, narrow lead and 115-112, but each competitor were relentless and brave, all the qualities their inspiration the legend represented.
