British light heavyweight Buatsi earned a win yet underwhelming debut to his career under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's work rate appeared to earn him a judges' verdict in a close 10-round fight at the boxing arena.
But Parker found success, landing some telling individual punches in a contest where the techniques never quite clicked, with excessive holding spoiling the fight.
The scoring judges judged it 96-94 in favor to the winner, with the third a 95-95 draw.
"It wasn't my greatest performance but what defines champions? They prevail," Buatsi said in the ring as he was greeted with negative reactions from the spectators.
The Londoner got back to winning after his maiden career setback - an memorable fight with his previous opponent in last winter - to claim a 20th career victory.
Various analysts thought Parker, 31, deserved the victory but the British boxer endured the second loss of his 28-match career.
The Champion Secures Win in Uneventful Affair
The fighter's advancement has plateaued in the past few years through contract problems and extended periods of inactivity.
Earlier forecast as the next star of British boxing, he now stands trying to fulfil that early promise - possibly in what feels like a ultimate opportunity - under new promotional backing.
He started carefully, with both competitors willing to overcommit in a cagey early stages.
Parker, whose single earlier setback came at super middleweight against his former opponent in 2022 after damaging his hand in the contest, was competing in only his second appearance at light-heavyweight.
By the fight's midpoint, the underdog had made it competitive, causing problems for the favorite, who operated with a solid defense to narrowly win the cleaner stanzas.
The crowd's patience decreased during frequent holding, with the challenger slipping to the canvas several times during close exchanges.
Parker closed well, but many rounds were difficult to score. Nevertheless, former world champion the boxing analyst - commentating on Dazn - had it particularly wide for Parker, scoring for Buatsi just one round.
The victor progresses, but with the likes of power puncher Benavidez, and Russian champion Dmitry Bivol leading the category, the path to championship success - based on this performance - still appears challenging.
Ricky Hatton Homage Motivates Manchester Derby Contest
In the secondary fight, Lyndon Arthur overcame Bradley Rea by close verdict in a terrific local rivalry contest to win the European light-heavyweight belt.
The arena fell silent for a extremely emotional memorial prior to the bout as a bell ceremony tolled in remembrance of local hero Ricky Hatton, who succumbed at age 46 in recently.
It was the first major televised boxing event in the urban center since Hatton's passing. As the final chime disappeared, the stillness became a roar of "only one Hatton".
The action during the bout matched the sentiment. Arthur was floored by a left hook directly to the face in the early round, rising shakily to his feet on the eight count.
But the red-colored shorts of Arthur soon came forward again. Across the ring, the opponent - in competitive blue - was marked up quickly, heavy swelling appearing under his face as the duo traded punishing blows.
Rea, 27, landed power punches but Arthur, thirty-four, responded in kind with stinging uppercuts as the intensity rarely dropped over twelve rounds.
The victor barely secured the decision with judges scorecards of even score, narrow lead and wider gap, but both fighters were relentless and brave, everything their idol the legend embodied.
