British light heavyweight Buatsi made a winning yet disappointing start to his career under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's work rate likely gained him a majority decision in a tight 10-round contest at the Co-op Live Arena.
But the challenger landed significant shots, connecting with powerful single shots in a contest where the approaches never quite clicked, with frequent clinching marring the action.
Two judges had it 96-94 in favor to the winner, with the remaining official a 95-95.
"It wasn't my best performance but what defines champions? They prevail," the victor stated in the ringside as he was greeted with negative reactions from the crowd.
The Londoner bounced back after his initial pro loss - an memorable fight with Callum Smith in last winter - to secure a 20th pro success.
Some ringside observers felt the challenger, thirty-one, should have been awarded the decision but the contender suffered the second loss of his 28-match career.
The Champion Claims Win in Uneventful Encounter
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 UK boxing, he now is positioned trying to realize his potential - possibly in what feels like a final roll of the dice - under recent support.
He opened tentatively, with neither fighter prepared to take risks in a tentative early stages.
Parker, whose sole prior loss came at super-middleweight against his former opponent in 2022 after damaging his hand during the bout, was participating in his second bout at light heavyweight.
By the fight's midpoint, the underdog had stayed in contention, causing problems for Buatsi, who utilized a tight guard to edge the cleaner stanzas.
The spectators' acceptance wore thin during regular grabbing, with Parker falling to the floor various instances during grappling.
The underdog finished the stronger, but many rounds were hard to judge. Nevertheless, previous titleholder Carl Frampton - providing analysis for Dazn - had it strongly leaning for Parker, awarding Buatsi just a single round.
The victor progresses, but with the likes of hard hitter Benavidez, and Russian star Bivol ruling the division, the path to championship success - judging by this showing - still appears challenging.
Ricky Hatton Homage Fuels Local Rivalry Fight
In the supporting bout, Arthur overcame Rea by close verdict in a outstanding local rivalry battle to secure the continental belt.
The venue grew hushed for a extremely emotional homage ahead of the contest as 10 bells rang in remembrance of local hero Hatton, who succumbed aged 46 in recently.
It was the first major televised boxing event in the urban center since Hatton's passing. As the final chime ended, the stillness became a roar of "there's just one Ricky Hatton".
The action within the ring matched the emotion. Arthur was sent to the canvas by a significant blow directly to the face in the initial stage, getting up unsteadily to his feet on the count of eight.
But the Manchester United-red equipment of Arthur soon came forward again. On the opposite side, Rea - in rival City blue - was damaged quickly, heavy swelling appearing under his orbital area as the fighters traded hard punches.
The challenger, twenty-seven, landed power punches but Arthur, thirty-four, matched the intensity with stinging uppercuts as the action rarely dropped over twelve rounds.
Arthur edged the verdict with judges scorecards of even score, narrow lead and wider gap, but each competitor were persistent and courageous, everything their inspiration the legend stood for.
