UK light-heavyweight Buatsi made a winning yet underwhelming debut to life under new promoters with a decision win over Parker in Manchester.
The Olympic medal winner's activity level seemingly secured him a judges' verdict in a close ten-round fight at the Manchester venue.
But Parker landed significant shots, connecting with powerful individual punches in a fight where the styles didn't complement each other, with constant grabbing disrupting the fight.
The scoring judges judged it 96-94 in favor to Buatsi, with the remaining official a even score.
"It wasn't my finest showing but what do champions do? They win," the victor said in the ring as he was faced disapproval from the crowd.
The British fighter bounced back after his initial pro loss - an thrilling contest with Callum Smith in February - to claim a 20th career victory.
Some ringside observers believed the challenger, thirty-one, should have been awarded the victory but the Derbyshire fighter suffered the second defeat of his 28-fight professional journey.
The Champion Claims Win in Uneventful Encounter
The fighter's advancement has slowed in recent times through management issues and long spells of lack of fights.
Previously predicted as the rising talent of the British fight scene, he now finds himself trying to fulfil that early promise - perhaps in what feels like a ultimate opportunity - under fresh management.
He opened tentatively, with both competitors ready to take risks in a calculated initial rounds.
The challenger, whose sole prior loss came at the lower weight class against his former opponent in two years ago after breaking his hand mid-fight, was making just his second bout at light heavyweight.
By the middle rounds, Parker had kept it close, creating difficulties for Buatsi, who utilized a solid defense to barely take the better periods.
The audience's tolerance diminished amid constant clinching, with Parker going down several times during close exchanges.
Parker closed well, but several stanzas were hard to judge. Nevertheless, previous titleholder Carl Frampton - providing analysis for Dazn - had it particularly wide for Parker, awarding Buatsi just only one stanza.
Buatsi advances, but with the caliber of hard hitter David Benavidez, and Russian star Dmitry Bivol ruling the division, the journey to titles - judging by this showing - still seems difficult.
Ricky Hatton Memorial Inspires Manchester Derby Contest
In the co-main event, Arthur overcame Bradley Rea by split-decision in a excellent all-Manchester fight to capture the continental title.
The stadium became quiet for a profoundly moving tribute prior to the bout as 10 bells tolled in memory of the city's native Ricky Hatton, who succumbed 46 years old in recently.
It was the premier televised boxing event in the location since Hatton's passing. As the concluding sound disappeared, the stillness transitioned to a shout of "there's only one Hatton".
The action inside the ropes matched the sentiment. The eventual winner was sent to the canvas by a powerful punch squarely on the jaw in the initial stage, rising shakily to his position on the referee's count.
But the traditional red equipment of the fighter soon advanced again. On the opposite side, the opponent - in opposing blue - was injured soon, noticeable damage appearing under his eye as the fighters landed powerful shots.
The challenger, twenty-seven, connected with strong blows but the champion, thirty-four, responded in kind with effective uppercuts as the pace hardly diminished over twelve rounds.
The victor edged the verdict with official scores of 114-114, close margin and wider gap, but each competitor were persistent and courageous, exactly what their idol the legend represented.
