UK light-heavyweight Buatsi secured a victory yet underwhelming debut to life under new promoters with a decision win over Parker in Manchester.
The Olympic bronze medallist's activity level likely gained him a majority decision in a close ten-round fight at the Co-op Live Arena.
But the challenger had his moments, delivering effective individual punches in a match where the techniques failed to mesh, with frequent clinching spoiling the action.
The officials judged it 96-94 to Buatsi, with the remaining official a 95-95.
"It wasn't my best performance but how do champions respond? They succeed," the victor said in the ring as he was faced boos from the audience.
The Londoner returned to winning ways after his maiden career setback - an memorable fight with his previous opponent in last winter - to secure a 20th pro success.
Various analysts thought Parker, thirty-one, deserved the victory but the Derbyshire fighter suffered the second setback of his 28-match professional journey.
The Champion Claims Triumph in Slow Affair
Buatsi's progress has plateaued in recent times through contract problems and long spells of lack of fights.
Once tipped as the next star of British boxing, he now finds himself trying to realize his potential - possibly in what appears to be a ultimate opportunity - under fresh management.
He began cautiously, with both competitors willing to take risks in a tentative early stages.
The challenger, whose single earlier setback came at the lower weight class against John Ryder in two years ago after breaking his hand in the contest, was participating in his second appearance at light heavyweight.
By the middle rounds, the underdog had kept it close, creating difficulties for Buatsi, who operated with a protective stance to edge the better stanzas.
The spectators' acceptance diminished amid regular grabbing, with Parker falling to the floor multiple occasions during close exchanges.
The underdog ended stronger, but several stanzas were challenging to assess. Although, former world champion the boxing analyst - working with the broadcast network - had it strongly leaning for Parker, scoring for Buatsi just one round.
The victor moves on, but with the presence of heavy hitter David Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the path to championship success - based on this performance - still looks steep.
Ricky Hatton Tribute Fuels Manchester Derby Fight
In the supporting bout, Lyndon Arthur beat Bradley Rea by split-decision in a terrific Manchester vs Manchester contest to win the continental title.
The venue grew hushed for a profoundly moving memorial prior to the bout as a bell ceremony rang in remembrance of the city's native Hatton, who died aged 46 in September.
It was the first major televised boxing event in the city since Hatton's death. As the last bell disappeared, the stillness became a chant of "only one Hatton".
The action within the ring reflected the sentiment. The eventual winner was floored by a left hook squarely on the jaw in the initial stage, getting up unsteadily to his feet on the eight count.
But the red-colored shorts of the fighter soon came forward again. On the opposite side, the opponent - in opposing blue - was injured soon, heavy swelling appearing under his face as the duo traded powerful shots.
Rea, twenty-seven, connected with strong blows but Arthur, 34, matched the intensity with effective uppercuts as the action seldom decreased over the full distance.
The victor narrowly won the verdict with judges scorecards of even score, close margin and 115-112, but both fighters were relentless and full of heart, everything their hero the legend represented.
