UK light heavyweight Buatsi secured a victory yet unimpressive start to life under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic bronze medallist's activity level seemingly secured him a split decision in a competitive ten-round contest at the Co-op Live Arena.
But the challenger landed significant shots, landing some telling clean blows in a match where the approaches didn't complement each other, with frequent clinching spoiling the fight.
The officials judged it 96-94 for to Buatsi, with the third a 95-95 draw.
"It wasn't my finest performance but what do champions do? They prevail," the victor said in the ring as he was faced negative reactions from the spectators.
The Londoner returned to winning ways after his initial pro loss - an memorable fight with Callum Smith in February - to claim a twentieth professional win.
Several boxing experts felt Parker, 31, should have been awarded the decision but the British boxer experienced the second setback of his 28-match career.
The Champion Secures Win in Dull Encounter
The fighter's advancement has stalled in recent times through contract problems and considerable time of inactivity.
Earlier forecast as the next star of British boxing, he now finds himself trying to fulfil that early promise - maybe in what seems like a last chance - under recent support.
He started carefully, with both competitors willing to take risks in a tentative early stages.
Parker, whose only previous defeat came at super-middleweight against his former opponent in two years ago after damaging his hand during the bout, was participating in his second bout at the higher weight class.
By the middle rounds, Parker had stayed in contention, causing problems for Buatsi, who worked behind a protective stance to barely take the cleaner periods.
The crowd's patience wore thin because of frequent holding, with Parker falling to the floor multiple occasions during clinches.
The underdog finished the stronger, but multiple periods were challenging to assess. However, previous titleholder the boxing analyst - working with Dazn - had it significantly in favor for the challenger, giving Buatsi just a single round.
The victor moves on, but with the caliber of power puncher David Benavidez, and Russian champion Dmitry Bivol ruling the division, the journey to titles - on this evidence - still looks steep.
Hatton Homage Fuels Manchester Derby Contest
In the co-main event, Arthur overcame Bradley Rea by split-decision in a outstanding local rivalry contest to secure the European light heavyweight belt.
The stadium fell silent for a profoundly moving tribute before the fight as 10 bells rang in honor of the city's native Hatton, who passed away at age 46 in last month.
It was the initial significant broadcast match card in the location since Hatton's death. As the final chime ended, the silence gave way to a roar of "there's just one Hatton".
The fighting within the ring matched the feeling. Arthur was knocked down by a significant blow directly to the jaw in the early round, rising shakily to his feet on the referee's count.
But the red-colored shorts of the fighter soon came forward again. On the opposite side, Rea - in opposing blue - was marked up early, heavy swelling appearing under his eye as the pair landed punishing blows.
The challenger, twenty-seven, delivered effective strikes but Arthur, thirty-four, matched the intensity with effective uppercuts as the action hardly diminished over the full distance.
The victor edged the result with judges scorecards of even score, 115-113 and 115-112, but each competitor were relentless and courageous, exactly what their hero the legend embodied.
