British light-heavyweight Buatsi earned a win yet underwhelming beginning to his career under recent management with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's work rate likely gained him a majority decision in a competitive 10-round contest at the Co-op Live Arena.
But the challenger had his moments, connecting with powerful clean blows in a match where the techniques failed to mesh, with constant grabbing spoiling the contest.
Two judges scored it 96-94 for to the winner, with the remaining official a 95-95 draw.
"It wasn't my best showing but what do champions do? They prevail," Buatsi stated in the ring as he was greeted with disapproval from the crowd.
The British fighter returned to winning ways after his initial pro loss - an epic battle with Callum Smith in February - to claim a twentieth professional win.
Several boxing experts felt Parker, thirty-one, should have been awarded the win but the Derbyshire fighter experienced the second setback of his 28-match career.
The Champion Claims Triumph in Dull Encounter
The fighter's advancement has plateaued in the past few years through contract problems and long spells of inactivity.
Earlier forecast as the next star of UK boxing, he now is positioned trying to fulfil that early promise - possibly in what seems like a final roll of the dice - under fresh management.
He began cautiously, with neither fighter ready to take risks in a tentative opening few rounds.
Parker, whose sole prior loss came at super-middleweight against John Ryder in two years ago after suffering a hand injury during the bout, was making just his second bout at the higher weight class.
By the middle rounds, the underdog had kept it close, causing problems for Buatsi, who worked behind a solid defense to barely take the cleaner rounds.
The crowd's patience decreased amid regular grabbing, with Parker falling to the floor multiple occasions during clinches.
Parker finished the stronger, but many rounds were challenging to assess. However, ex-champion Carl Frampton - providing analysis for Dazn - had it particularly wide for Parker, giving Buatsi just a single round.
Buatsi moves on, but with the likes of heavy hitter Benavidez, and Eastern European fighter Bivol ruling the division, the journey to titles - on this evidence - still appears challenging.
Ricky Hatton Homage Fuels Local Rivalry Contest
In the supporting bout, Arthur overcame Rea by close verdict in a outstanding local rivalry contest to win the European light-heavyweight belt.
The stadium grew hushed for a profoundly moving memorial ahead of the contest as ten bells rang in honor of local hero Hatton, who succumbed at age 46 in September.
It was the initial significant broadcast match card in the city since Hatton's death. As the final chime faded, the quiet gave way to a chant of "only one Hatton".
The action during the bout reflected the emotion. Arthur was floored by a powerful punch directly to the chin in the initial stage, standing uncertainly to his position on the count of eight.
But the red-colored shorts of Arthur soon advanced again. Across the ring, the opponent - in competitive blue - was damaged quickly, noticeable damage developing under his eye as the fighters traded punishing blows.
Rea, twenty-seven, connected with strong blows but Arthur, 34, responded in kind with stinging uppercuts as the intensity hardly diminished over the full distance.
The victor barely secured the result with judges scorecards of even score, narrow lead and wider gap, but both fighters were persistent and courageous, everything their inspiration Hatton represented.
