UK light-heavyweight Buatsi made a winning yet disappointing debut to his career under recent management with a points victory over Parker in Manchester.
The Olympic bronze medallist's work rate appeared to earn him a split decision in a close 10-round bout at the Co-op Live Arena.
But the challenger had his moments, connecting with powerful individual punches in a match where the styles never quite clicked, with constant grabbing disrupting the contest.
The scoring judges judged it 96-94 for to the winner, with the third a 95-95.
"It wasn't my best showing but what defines champions? They prevail," the victor said in the ring as he was met by boos from the crowd.
The Londoner returned to winning ways after his initial pro loss - an epic battle with his previous opponent in February - to earn a 20th career victory.
Several boxing experts thought the challenger, 31, should have been awarded the decision but the British boxer experienced the second setback of his 28-fight career.
Buatsi Secures Triumph in Uneventful Match
Buatsi's progress has plateaued in recent times through contract problems and extended periods of inactivity.
Previously predicted as the rising talent of British boxing, he now stands trying to realize his potential - possibly in what seems like a final roll of the dice - under new promotional backing.
He opened tentatively, with the two boxers willing to take risks in a tentative opening few rounds.
Parker, whose sole prior loss came at super-middleweight against his former opponent in 2022 after breaking his hand mid-fight, was participating in his second bout at the higher weight class.
By the fight's midpoint, Parker had stayed in contention, causing problems for the favorite, who utilized a protective stance to narrowly win the more effective periods.
The audience's tolerance diminished amid frequent holding, with the challenger falling to the floor several times during grappling.
Parker closed well, but several stanzas were hard to judge. However, former world champion the boxing analyst - working with the broadcast network - had it significantly in favor for Parker, awarding Buatsi just only one stanza.
Buatsi advances, but with the likes of heavy hitter Benavidez, and Russian star Dmitry Bivol leading the category, the road to world glory - judging by this showing - still appears challenging.
Hatton Memorial Motivates All-Manchester Fight
In the co-main event, Lyndon Arthur defeated Bradley Rea by split-decision in a excellent Manchester vs Manchester contest to win the European light-heavyweight championship.
The stadium became quiet for a extremely emotional tribute prior to the bout as ten bells sounded in honor of the city's native Hatton, who passed away at age 46 in recently.
It was the premier televised match card in the location since the passing of Hatton. As the concluding sound faded, the silence gave way to a roar of "there's just one Hatton".
The competition during the bout matched the feeling. The eventual winner was sent to the canvas by a powerful punch directly to the chin in the early round, getting up unsteadily to his position on the referee's count.
But the Manchester United-red shorts of the fighter soon came forward again. Across the ring, Rea - in rival City blue - was damaged quickly, heavy swelling appearing under his face as the pair traded powerful shots.
Rea, 27, connected with strong blows but the champion, thirty-four, matched the intensity with stinging uppercuts as the action rarely dropped over the full distance.
Arthur edged the result with official scores of draw, narrow lead and clearer margin, but both fighters were persistent and brave, exactly what their inspiration the legend represented.
