UK light-heavyweight Buatsi secured a victory yet disappointing start to life under new promoters with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's work rate seemingly secured him a judges' verdict in a close ten-round fight at the boxing arena.
But Parker had his moments, connecting with powerful single shots in a match where the approaches didn't complement each other, with constant grabbing marring the action.
Two judges scored it 96-94 in favor to Buatsi, with the third a even score.
"It wasn't my finest performance but how do champions respond? They succeed," the victor stated in the ring as he was met by negative reactions from the spectators.
The British fighter got back to winning after his initial pro loss - an epic battle with Callum Smith in February - to claim a 20th pro success.
Various analysts felt the challenger, thirty-one, should have been awarded the decision but the contender suffered the second setback of his 28-match professional journey.
Buatsi Secures Victory in Dull Affair
The fighter's advancement has slowed in the past few years through promotional disputes and long spells of inactivity.
Once tipped as the next star of British boxing, he now finds himself trying to achieve expectations - possibly in what appears to be a final roll of the dice - under new promotional backing.
He opened tentatively, with neither fighter ready to overcommit in a cagey opening few rounds.
The challenger, whose sole prior loss came at super middleweight against his former opponent in two years ago after damaging his hand mid-fight, was competing in only his second appearance at the higher weight class.
By the fight's midpoint, Parker had made it competitive, making life awkward for Buatsi, who worked behind a protective stance to barely take the cleaner stanzas.
The spectators' acceptance diminished amid constant clinching, with Parker going down several times during grappling.
The underdog ended stronger, but multiple periods were challenging to assess. Although, former world champion Carl Frampton - providing analysis for Dazn - had it significantly in favor for Parker, scoring for Buatsi just only one stanza.
Buatsi progresses, but with the likes of hard hitter David Benavidez, and Russian star Dmitry Bivol ruling the division, the path to championship success - on this evidence - still looks steep.
Hatton Memorial Fuels Manchester Derby Fight
In the supporting bout, Lyndon Arthur overcame Bradley Rea by close verdict in a terrific all-Manchester battle to secure the continental title.
The venue grew hushed for a profoundly moving memorial ahead of the contest as ten bells sounded in honor of Manchester's own Ricky Hatton, who passed away at age 46 in September.
It was the premier televised fight night in the location since Hatton's passing. As the final chime ended, the quiet transitioned to a roar of "there's just one Hatton".
The competition inside the ropes equaled the sentiment. The eventual winner was sent to the canvas by a significant blow squarely on the chin in the second round, getting up unsteadily to his position on the referee's count.
But the Manchester United-red shorts of the fighter soon pressed ahead again. Across the ring, the opponent - in competitive blue - was marked up early, heavy swelling developing under his orbital area as the pair traded punishing blows.
The challenger, twenty-seven, delivered effective strikes but Arthur, 34, fought fire with fire with stinging uppercuts as the intensity hardly diminished over twelve rounds.
The victor barely secured the verdict with scoring tallies of draw, close margin and wider gap, but the two athletes were relentless and courageous, all the qualities their hero the legend embodied.
