British light heavyweight Buatsi earned a win yet disappointing start to his career under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic bronze medallist's work rate seemingly secured him a split decision in a competitive ten-round bout at the boxing arena.
But Parker had his moments, delivering effective clean blows in a contest where the approaches didn't complement each other, with frequent clinching spoiling the action.
The officials scored it 96-94 to Buatsi, with the third a 95-95.
"It wasn't my best showing but what do champions do? They win," the victor said in the arena as he was met by negative reactions from the audience.
The Londoner got back to winning after his maiden career setback - an thrilling contest with Callum Smith in February - to claim a 20th career victory.
Some ringside observers felt the challenger, 31, deserved the decision but the contender endured the second loss of his 28-match professional journey.
Buatsi Secures Victory in Dull Match
Buatsi's progress has slowed in the past few years through contract problems and long spells of lack of fights.
Earlier forecast as the rising talent of UK boxing, he now finds himself trying to realize his potential - perhaps in what seems like a ultimate opportunity - under new promotional backing.
He started carefully, with both competitors ready to take risks in a tentative initial rounds.
Parker, whose only previous defeat came at the lower weight class against his former opponent in 2022 after breaking his hand during the bout, was competing in only his second fight at the higher weight class.
By the middle rounds, Parker had made it competitive, making life awkward for Buatsi, who operated with a solid defense to barely take the better stanzas.
The spectators' acceptance diminished during regular grabbing, with Parker falling to the floor multiple occasions during grappling.
The underdog closed well, but many rounds were difficult to score. However, previous titleholder Carl Frampton - providing analysis for Dazn - had it significantly in favor for Parker, scoring for Buatsi just a single round.
Buatsi moves on, but with the presence of hard hitter David Benavidez, and Russian star Dmitry Bivol ruling the division, the road to world glory - based on this performance - still seems difficult.
Hatton Memorial Fuels Local Rivalry Fight
In the supporting bout, Lyndon Arthur defeated Bradley Rea by split-decision in a outstanding all-Manchester fight to secure the European light-heavyweight belt.
The venue grew hushed for a extremely emotional tribute ahead of the contest as 10 bells tolled in remembrance of local hero Ricky Hatton, who died at age 46 in last month.
It was the premier televised match card in the urban center since Hatton's passing. As the last bell faded, the silence transitioned to a roar of "there's only one Hatton".
The competition inside the ropes matched the feeling. Arthur was knocked down by a powerful punch squarely on the jaw in the initial stage, getting up unsteadily to his feet on the referee's count.
But the red-colored shorts of Arthur soon advanced again. From the other corner, the opponent - in rival City blue - was injured early, significant bruising forming under his orbital area as the duo exchanged hard punches.
The challenger, twenty-seven, landed power punches but Arthur, 34, responded in kind with stinging uppercuts as the action hardly diminished over the full distance.
Arthur narrowly won the verdict with scoring tallies of 114-114, 115-113 and wider gap, but both fighters were persistent and brave, all the qualities their hero Hatton represented.
