British light-heavyweight Buatsi made a winning yet disappointing debut to his career under recent management with a points victory over Parker in the Manchester arena.
The Olympic medal winner's work rate likely gained him a split decision in a competitive 10-round bout at the Co-op Live Arena.
But Parker had his moments, connecting with powerful single shots in a contest where the styles failed to mesh, with frequent clinching spoiling the fight.
The scoring judges judged it 96-94 for to Buatsi, with the final judge a 95-95 draw.
"It wasn't my finest showing but what defines champions? They prevail," the victor said in the ringside as he was faced negative reactions from the spectators.
The British fighter bounced back after his maiden career setback - an epic battle with his previous opponent in February - to secure a twentieth professional win.
Some ringside observers believed the challenger, thirty-one, should have been awarded the victory but the British boxer suffered the second loss of his 28-fight career.
Buatsi Claims Win in Uneventful Match
His career development has plateaued in recent times through promotional disputes and extended periods of inactivity.
Earlier forecast as the next star of UK boxing, he now finds himself trying to fulfil that early promise - perhaps in what feels like a final roll of the dice - under recent support.
He began cautiously, with the two boxers willing to overcommit in a cagey initial rounds.
The challenger, whose single earlier setback came at super middleweight against his former opponent in 2022 after damaging his hand during the bout, was making just his second bout at light heavyweight.
By the fight's midpoint, Parker had stayed in contention, causing problems for the favorite, who operated with a tight guard to narrowly win the cleaner stanzas.
The spectators' acceptance decreased amid frequent holding, with the challenger going down several times during close exchanges.
The underdog ended stronger, but several stanzas were difficult to score. However, previous titleholder Carl Frampton - working with the broadcast network - had it significantly in favor for Parker, scoring for Buatsi just only one stanza.
The victor progresses, but with the presence of power puncher Benavidez, and Russian champion Bivol dominating the weight class, the journey to titles - judging by this showing - still seems difficult.
Ricky Hatton Tribute Inspires All-Manchester Contest
In the co-main event, Lyndon Arthur overcame Bradley Rea by divided decision in a excellent Manchester vs Manchester battle to secure the continental belt.
The stadium became quiet for a deeply poignant memorial ahead of the contest as 10 bells tolled in remembrance of local hero Ricky Hatton, who died 46 years old in September.
It was the first major televised match card in the urban center since Hatton's death. As the last bell disappeared, the stillness became a roar of "there's only one Hatton".
The competition within the ring equaled the sentiment. Arthur was floored by a left hook flush on the face in the initial stage, getting up unsteadily to his position on the referee's count.
But the traditional red shorts of the fighter soon advanced again. On the opposite side, the opponent - in rival City blue - was injured early, heavy swelling forming under his orbital area as the pair landed powerful shots.
The challenger, twenty-seven, delivered effective strikes but Arthur, 34, responded in kind with powerful upward punches as the action seldom decreased over 12 rounds.
The victor narrowly won the result with judges scorecards of even score, narrow lead and 115-112, but the two athletes were relentless and courageous, exactly what their hero Hatton stood for.
