UK light-heavyweight Buatsi secured a victory yet underwhelming start to life under recent management with a decision win over Parker in Manchester.
The Olympic medal winner's work rate likely gained him a judges' verdict in a competitive 10-round contest at the boxing arena.
But the challenger landed significant shots, connecting with powerful individual punches in a fight where the styles failed to mesh, with frequent clinching spoiling the contest.
Two judges judged it 96-94 to the winner, with the final judge a 95-95.
"It wasn't my finest showing but what do champions do? They win," Buatsi stated in the ring as he was met by boos from the spectators.
The British fighter returned to winning ways after his first professional defeat - an epic battle with his previous opponent in last winter - to claim a twentieth professional win.
Several boxing experts felt Parker, thirty-one, should have been awarded the decision but the British boxer suffered the second loss of his 28-fight professional journey.
The Champion Earns Win in Uneventful Encounter
His career development has stalled in recent times through contract problems and extended periods of inactivity.
Earlier forecast as the rising talent of the British fight scene, he now stands trying to fulfil that early promise - perhaps in what seems like a last chance - under fresh management.
He began cautiously, with neither fighter willing to be aggressive in a cagey initial rounds.
The challenger, whose sole prior loss came at super-middleweight against John Ryder in 2022 after damaging his hand in the contest, was competing in only his second bout at light-heavyweight.
By the halfway stage, the underdog had kept it close, making life awkward for Buatsi, who utilized a protective stance to barely take the cleaner periods.
The crowd's patience diminished because of regular grabbing, with the challenger going down various instances during close exchanges.
Parker finished the stronger, but multiple periods were challenging to assess. However, previous titleholder Carl Frampton - commentating on the broadcast network - had it strongly leaning for the challenger, giving Buatsi just only one stanza.
Buatsi moves on, but with the presence of heavy hitter Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the road to world glory - on this evidence - still appears challenging.
Ricky Hatton Homage Fuels Manchester Derby Fight
In the supporting bout, Lyndon Arthur defeated Rea by close verdict in a excellent local rivalry fight to capture the European light-heavyweight title.
The stadium grew hushed for a deeply poignant homage prior to the bout as 10 bells tolled in memory of local hero Hatton, who succumbed 46 years old in last month.
It was the initial significant broadcast match card in the city since Hatton's death. As the concluding sound ended, the quiet gave way to a roar of "only one Hatton".
The action inside the ropes equaled the emotion. Arthur was floored by a powerful punch flush on the face in the early round, standing uncertainly to his footing on the referee's count.
But the red-colored attire of Arthur soon advanced again. From the other corner, the opponent - in rival City blue - was injured quickly, heavy swelling forming under his face as the pair landed powerful shots.
The challenger, 27, delivered effective strikes but the champion, thirty-four, matched the intensity with stinging uppercuts as the action seldom decreased over the full distance.
The victor narrowly won the result with judges scorecards of 114-114, close margin and 115-112, but each competitor were persistent and full of heart, exactly what their idol Hatton stood for.
