British light-heavyweight Buatsi made a winning yet unimpressive start to life under recent management with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's work rate seemingly secured him a judges' verdict in a tight ten-round contest at the Co-op Live Arena.
But Parker found success, landing some telling individual punches in a fight where the approaches never quite clicked, with frequent clinching spoiling the action.
The scoring judges had it 96-94 in favor to the winner, with the third a 95-95.
"It wasn't my best performance but what defines champions? They win," Buatsi said in the arena as he was faced disapproval from the crowd.
The British fighter got back to winning after his initial pro loss - an memorable fight with Callum Smith in February - to claim a 20th pro success.
Various analysts felt Parker, 31, merited the decision but the Derbyshire fighter suffered the second loss of his 28-bout professional journey.
The Champion Claims Triumph in Slow Encounter
His career development has stalled in recent times through management issues and extended periods of lack of fights.
Earlier forecast as the next star of the British fight scene, he now stands trying to fulfil that early promise - perhaps in what seems like a ultimate opportunity - under fresh management.
He began cautiously, with the two boxers ready to overcommit in a cagey opening few 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 participating in his second appearance at light-heavyweight.
By the fight's midpoint, Parker had made it competitive, creating difficulties for the favorite, who utilized a tight guard to narrowly win the more effective stanzas.
The audience's tolerance decreased during frequent holding, with the challenger falling to the floor various instances during grappling.
The underdog closed well, but multiple periods were challenging to assess. Nevertheless, ex-champion Carl Frampton - providing analysis for Dazn - had it strongly leaning for the challenger, giving Buatsi just only one stanza.
The victor moves on, but with the presence of heavy hitter David Benavidez, and Russian champion Bivol dominating the weight class, the journey to titles - on this evidence - still appears challenging.
Hatton Homage Motivates Local Rivalry Fight
In the secondary fight, Arthur defeated Rea by close verdict in a terrific Manchester vs Manchester contest to capture the continental championship.
The venue fell silent for a deeply poignant homage ahead of the contest as a bell ceremony rang in memory of Manchester's own Ricky Hatton, who passed away aged 46 in recently.
It was the initial significant broadcast boxing event in the urban center since Hatton's death. As the last bell disappeared, the quiet became a roar of "there's just one Ricky Hatton".
The action during the bout matched the sentiment. Arthur was knocked down by a left hook directly to the chin in the early round, getting up unsteadily to his position on the eight count.
But the red-colored equipment of the fighter soon pressed ahead again. On the opposite side, Rea - in competitive blue - was marked up early, noticeable damage developing under his face as the fighters landed powerful shots.
Rea, twenty-seven, connected with strong blows but Arthur, 34, matched the intensity with stinging uppercuts as the action rarely dropped over the full distance.
The victor barely secured the verdict with scoring tallies of draw, 115-113 and 115-112, but both fighters were determined and brave, all the qualities their idol Hatton represented.
