UK light heavyweight Buatsi earned a win yet underwhelming start to his career under recent management with a points victory over Parker in Manchester.
The Olympic bronze medallist's activity level seemingly secured him a majority decision in a close 10-round contest 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 frequent clinching marring the action.
Two judges had it 96-94 to the winner, with the remaining official a 95-95 draw.
"It wasn't my finest performance but what do champions do? They prevail," the victor commented in the ringside as he was greeted with boos from the audience.
The Londoner got back to winning after his initial pro loss - an memorable fight with his previous opponent in February - to earn a twentieth professional win.
Various analysts thought the challenger, 31, should have been awarded the win but the contender endured the second defeat of his 28-match professional journey.
Buatsi Secures Victory in Slow Match
The fighter's advancement has plateaued in recent years through promotional disputes and extended periods of lack of fights.
Previously predicted as the next star of UK boxing, he now finds himself trying to fulfil that early promise - maybe in what feels like a last chance - under recent support.
He opened tentatively, with neither fighter prepared to take risks in a tentative opening few rounds.
Parker, whose only previous defeat came at the lower weight class against John Ryder in two years ago after breaking his hand mid-fight, was making just his second fight at the higher weight class.
By the fight's midpoint, the underdog had kept it close, making life awkward for the favorite, who worked behind a tight guard to edge the more effective stanzas.
The crowd's patience wore thin amid regular grabbing, with Parker going down various instances during clinches.
Parker ended stronger, but many rounds were challenging to assess. However, former world champion Carl Frampton - providing analysis for the broadcast network - had it particularly wide for Parker, giving Buatsi just one round.
The victor advances, but with the presence of hard hitter David Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the journey to titles - on this evidence - still seems difficult.
Ricky Hatton Tribute Inspires Local Rivalry Fight
In the supporting bout, Lyndon Arthur beat Bradley Rea by split-decision in a outstanding all-Manchester contest to secure the European light-heavyweight championship.
The venue became quiet for a deeply poignant memorial before the fight as 10 bells tolled in memory of Manchester's own Ricky Hatton, who died 46 years old in last month.
It was the initial significant broadcast boxing event in the city since Hatton's passing. As the final chime disappeared, the silence transitioned to a chant of "only one Ricky Hatton".
The competition within the ring equaled the feeling. Arthur was floored by a left hook squarely on the chin in the second round, standing uncertainly to his position on the referee's count.
But the traditional red shorts of Arthur soon advanced again. From the other corner, Rea - in competitive blue - was injured soon, heavy swelling appearing under his eye as the fighters exchanged powerful shots.
The challenger, twenty-seven, landed power punches but the champion, 34, fought fire with fire with stinging uppercuts as the intensity seldom decreased over twelve rounds.
Arthur narrowly won the decision with judges scorecards of 114-114, narrow lead and wider gap, but the two athletes were relentless and full of heart, everything their idol Hatton represented.
