UK light heavyweight Buatsi made a winning yet disappointing start to his career under recent management with a decision win over Parker in the Manchester arena.
The Olympic medal winner's work rate likely gained him a judges' verdict in a tight ten-round contest at the Manchester venue.
But the challenger found success, landing some telling individual punches in a fight where the styles never quite clicked, with constant grabbing spoiling the fight.
Two judges scored it 96-94 to Buatsi, with the third a 95-95.
"It wasn't my finest showing but what defines champions? They win," Buatsi said in the ring as he was greeted with negative reactions from the crowd.
The Londoner got back to winning after his first professional defeat - an epic battle with Callum Smith in February - to secure a twentieth professional win.
Several boxing experts thought Parker, thirty-one, should have been awarded the win but the Derbyshire fighter experienced the second defeat of his 28-match professional journey.
Buatsi Earns Triumph in Dull Encounter
His career development has plateaued in recent years through management issues and considerable time of inactivity.
Earlier forecast as the future of UK boxing, he now is positioned trying to realize his potential - perhaps in what feels like a final roll of the dice - under new promotional backing.
He began cautiously, with the two boxers ready to take risks in a tentative initial rounds.
Parker, whose sole prior loss 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 bout at light-heavyweight.
By the fight's midpoint, Parker had kept it close, causing problems for Buatsi, who operated with a protective stance to narrowly win the more effective periods.
The crowd's patience wore thin during frequent holding, with the challenger falling to the floor several times during close exchanges.
The underdog ended stronger, but many rounds were hard to judge. However, ex-champion Carl Frampton - working with the broadcast network - had it strongly leaning for Parker, scoring for Buatsi just a single round.
The victor moves on, but with the caliber of power puncher David Benavidez, and Russian star Dmitry Bivol ruling the division, the journey to titles - on this evidence - still seems difficult.
Ricky Hatton Homage Inspires Local Rivalry Battle
In the secondary fight, Arthur beat Bradley Rea by close verdict in a terrific local rivalry fight to win the continental title.
The arena became quiet for a profoundly moving homage prior to the bout as a bell ceremony rang in memory of Manchester's own Ricky Hatton, who died at age 46 in September.
It was the premier televised fight night in the city since the passing of Hatton. As the concluding sound disappeared, the quiet transitioned to a shout of "there's just one Ricky Hatton".
The action within the ring reflected the feeling. Arthur was knocked down by a left hook flush on the face in the early round, rising shakily to his feet on the referee's count.
But the traditional red equipment of Arthur soon pressed ahead again. Across the ring, Rea - in rival City blue - was injured soon, heavy swelling appearing under his face as the fighters traded punishing blows.
Rea, twenty-seven, connected with strong blows but the champion, 34, responded in kind with stinging uppercuts as the action seldom decreased over the full distance.
The victor narrowly won the verdict with judges scorecards of draw, close margin and wider gap, but both fighters were relentless and full of heart, everything their hero the legend embodied.
