British light-heavyweight Buatsi secured a victory yet disappointing debut to life under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output likely gained him a judges' verdict in a close ten-round contest at the Manchester venue.
But Parker had his moments, landing some telling individual punches in a contest where the approaches didn't complement each other, with frequent clinching disrupting the fight.
The officials had it 96-94 in favor to the winner, with the third a even score.
"It wasn't my best showing but what defines champions? They win," Buatsi said in the arena as he was greeted with negative reactions from the spectators.
The British fighter bounced back after his maiden career setback - an epic battle with Callum Smith in last winter - to earn a 20th career victory.
Various analysts felt the challenger, thirty-one, deserved the victory but the contender experienced the second setback of his 28-match career.
The Champion Claims Win in Slow Encounter
Buatsi's progress has stalled in recent years through management issues and extended periods of lack of fights.
Previously predicted as the rising talent of UK boxing, he now stands trying to achieve expectations - maybe in what feels like a last chance - under new promotional backing.
He started carefully, with the two boxers ready to overcommit in a cagey early stages.
The challenger, whose sole prior loss came at super-middleweight against John Ryder in 2022 after breaking his hand during the bout, was making just his second appearance at the higher weight class.
By the middle rounds, the underdog had stayed in contention, making life awkward for the favorite, who utilized a solid defense to edge the better periods.
The spectators' acceptance decreased amid constant clinching, with Parker going down several times during close exchanges.
Parker ended stronger, but many rounds were difficult to score. Nevertheless, previous titleholder Carl Frampton - providing analysis for Dazn - had it particularly wide for Parker, awarding Buatsi just one round.
Buatsi moves on, but with the likes of hard hitter David Benavidez, and Russian champion Bivol ruling the division, the journey to titles - judging by this showing - still appears challenging.
Ricky Hatton Tribute Fuels All-Manchester Fight
In the secondary fight, Arthur defeated Rea by close verdict in a terrific Manchester vs Manchester battle to capture the European light-heavyweight title.
The stadium fell silent for a profoundly moving memorial prior to the bout as a bell ceremony tolled in memory of local hero Hatton, who died at age 46 in last month.
It was the first major televised boxing event in the city since the passing of Hatton. As the final chime ended, the quiet transitioned to a chant of "there's only one Ricky Hatton".
The action inside the ropes matched the feeling. The eventual winner was knocked down by a powerful punch flush on the jaw in the early round, getting up unsteadily to his footing on the referee's count.
But the red-colored attire of the fighter soon came forward again. Across the ring, Rea - in opposing blue - was marked up quickly, significant bruising appearing under his eye as the fighters exchanged powerful shots.
The challenger, twenty-seven, connected with strong blows but the champion, thirty-four, responded in kind with powerful upward punches as the pace hardly diminished over twelve rounds.
Arthur edged the result with judges scorecards of draw, 115-113 and wider gap, but the two athletes were persistent and courageous, exactly what their idol Hatton embodied.
